--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/env/lib/python3.7/site-packages/ruamel/yaml/scanner.py	Sat May 02 07:14:21 2020 -0400
@@ -0,0 +1,1977 @@
+# coding: utf-8
+
+from __future__ import print_function, absolute_import, division, unicode_literals
+
+# Scanner produces tokens of the following types:
+# STREAM-START
+# STREAM-END
+# DIRECTIVE(name, value)
+# DOCUMENT-START
+# DOCUMENT-END
+# BLOCK-SEQUENCE-START
+# BLOCK-MAPPING-START
+# BLOCK-END
+# FLOW-SEQUENCE-START
+# FLOW-MAPPING-START
+# FLOW-SEQUENCE-END
+# FLOW-MAPPING-END
+# BLOCK-ENTRY
+# FLOW-ENTRY
+# KEY
+# VALUE
+# ALIAS(value)
+# ANCHOR(value)
+# TAG(value)
+# SCALAR(value, plain, style)
+#
+# RoundTripScanner
+# COMMENT(value)
+#
+# Read comments in the Scanner code for more details.
+#
+
+from ruamel.yaml.error import MarkedYAMLError
+from ruamel.yaml.tokens import *  # NOQA
+from ruamel.yaml.compat import utf8, unichr, PY3, check_anchorname_char, nprint  # NOQA
+
+if False:  # MYPY
+    from typing import Any, Dict, Optional, List, Union, Text  # NOQA
+    from ruamel.yaml.compat import VersionType  # NOQA
+
+__all__ = ['Scanner', 'RoundTripScanner', 'ScannerError']
+
+
+_THE_END = '\n\0\r\x85\u2028\u2029'
+_THE_END_SPACE_TAB = ' \n\0\t\r\x85\u2028\u2029'
+_SPACE_TAB = ' \t'
+
+
+class ScannerError(MarkedYAMLError):
+    pass
+
+
+class SimpleKey(object):
+    # See below simple keys treatment.
+
+    def __init__(self, token_number, required, index, line, column, mark):
+        # type: (Any, Any, int, int, int, Any) -> None
+        self.token_number = token_number
+        self.required = required
+        self.index = index
+        self.line = line
+        self.column = column
+        self.mark = mark
+
+
+class Scanner(object):
+    def __init__(self, loader=None):
+        # type: (Any) -> None
+        """Initialize the scanner."""
+        # It is assumed that Scanner and Reader will have a common descendant.
+        # Reader do the dirty work of checking for BOM and converting the
+        # input data to Unicode. It also adds NUL to the end.
+        #
+        # Reader supports the following methods
+        #   self.peek(i=0)    # peek the next i-th character
+        #   self.prefix(l=1)  # peek the next l characters
+        #   self.forward(l=1) # read the next l characters and move the pointer
+
+        self.loader = loader
+        if self.loader is not None and getattr(self.loader, '_scanner', None) is None:
+            self.loader._scanner = self
+        self.reset_scanner()
+        self.first_time = False
+
+    @property
+    def flow_level(self):
+        # type: () -> int
+        return len(self.flow_context)
+
+    def reset_scanner(self):
+        # type: () -> None
+        # Had we reached the end of the stream?
+        self.done = False
+
+        # flow_context is an expanding/shrinking list consisting of '{' and '['
+        # for each unclosed flow context. If empty list that means block context
+        self.flow_context = []  # type: List[Text]
+
+        # List of processed tokens that are not yet emitted.
+        self.tokens = []  # type: List[Any]
+
+        # Add the STREAM-START token.
+        self.fetch_stream_start()
+
+        # Number of tokens that were emitted through the `get_token` method.
+        self.tokens_taken = 0
+
+        # The current indentation level.
+        self.indent = -1
+
+        # Past indentation levels.
+        self.indents = []  # type: List[int]
+
+        # Variables related to simple keys treatment.
+
+        # A simple key is a key that is not denoted by the '?' indicator.
+        # Example of simple keys:
+        #   ---
+        #   block simple key: value
+        #   ? not a simple key:
+        #   : { flow simple key: value }
+        # We emit the KEY token before all keys, so when we find a potential
+        # simple key, we try to locate the corresponding ':' indicator.
+        # Simple keys should be limited to a single line and 1024 characters.
+
+        # Can a simple key start at the current position? A simple key may
+        # start:
+        # - at the beginning of the line, not counting indentation spaces
+        #       (in block context),
+        # - after '{', '[', ',' (in the flow context),
+        # - after '?', ':', '-' (in the block context).
+        # In the block context, this flag also signifies if a block collection
+        # may start at the current position.
+        self.allow_simple_key = True
+
+        # Keep track of possible simple keys. This is a dictionary. The key
+        # is `flow_level`; there can be no more that one possible simple key
+        # for each level. The value is a SimpleKey record:
+        #   (token_number, required, index, line, column, mark)
+        # A simple key may start with ALIAS, ANCHOR, TAG, SCALAR(flow),
+        # '[', or '{' tokens.
+        self.possible_simple_keys = {}  # type: Dict[Any, Any]
+
+    @property
+    def reader(self):
+        # type: () -> Any
+        try:
+            return self._scanner_reader  # type: ignore
+        except AttributeError:
+            if hasattr(self.loader, 'typ'):
+                self._scanner_reader = self.loader.reader
+            else:
+                self._scanner_reader = self.loader._reader
+            return self._scanner_reader
+
+    @property
+    def scanner_processing_version(self):  # prefix until un-composited
+        # type: () -> VersionType
+        if hasattr(self.loader, 'typ'):
+            return self.loader.resolver.processing_version  # type: ignore
+        return self.loader.processing_version  # type: ignore
+
+    # Public methods.
+
+    def check_token(self, *choices):
+        # type: (Any) -> bool
+        # Check if the next token is one of the given types.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        if bool(self.tokens):
+            if not choices:
+                return True
+            for choice in choices:
+                if isinstance(self.tokens[0], choice):
+                    return True
+        return False
+
+    def peek_token(self):
+        # type: () -> Any
+        # Return the next token, but do not delete if from the queue.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        if bool(self.tokens):
+            return self.tokens[0]
+
+    def get_token(self):
+        # type: () -> Any
+        # Return the next token.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        if bool(self.tokens):
+            self.tokens_taken += 1
+            return self.tokens.pop(0)
+
+    # Private methods.
+
+    def need_more_tokens(self):
+        # type: () -> bool
+        if self.done:
+            return False
+        if not self.tokens:
+            return True
+        # The current token may be a potential simple key, so we
+        # need to look further.
+        self.stale_possible_simple_keys()
+        if self.next_possible_simple_key() == self.tokens_taken:
+            return True
+        return False
+
+    def fetch_comment(self, comment):
+        # type: (Any) -> None
+        raise NotImplementedError
+
+    def fetch_more_tokens(self):
+        # type: () -> Any
+        # Eat whitespaces and comments until we reach the next token.
+        comment = self.scan_to_next_token()
+        if comment is not None:  # never happens for base scanner
+            return self.fetch_comment(comment)
+        # Remove obsolete possible simple keys.
+        self.stale_possible_simple_keys()
+
+        # Compare the current indentation and column. It may add some tokens
+        # and decrease the current indentation level.
+        self.unwind_indent(self.reader.column)
+
+        # Peek the next character.
+        ch = self.reader.peek()
+
+        # Is it the end of stream?
+        if ch == '\0':
+            return self.fetch_stream_end()
+
+        # Is it a directive?
+        if ch == '%' and self.check_directive():
+            return self.fetch_directive()
+
+        # Is it the document start?
+        if ch == '-' and self.check_document_start():
+            return self.fetch_document_start()
+
+        # Is it the document end?
+        if ch == '.' and self.check_document_end():
+            return self.fetch_document_end()
+
+        # TODO: support for BOM within a stream.
+        # if ch == u'\uFEFF':
+        #     return self.fetch_bom()    <-- issue BOMToken
+
+        # Note: the order of the following checks is NOT significant.
+
+        # Is it the flow sequence start indicator?
+        if ch == '[':
+            return self.fetch_flow_sequence_start()
+
+        # Is it the flow mapping start indicator?
+        if ch == '{':
+            return self.fetch_flow_mapping_start()
+
+        # Is it the flow sequence end indicator?
+        if ch == ']':
+            return self.fetch_flow_sequence_end()
+
+        # Is it the flow mapping end indicator?
+        if ch == '}':
+            return self.fetch_flow_mapping_end()
+
+        # Is it the flow entry indicator?
+        if ch == ',':
+            return self.fetch_flow_entry()
+
+        # Is it the block entry indicator?
+        if ch == '-' and self.check_block_entry():
+            return self.fetch_block_entry()
+
+        # Is it the key indicator?
+        if ch == '?' and self.check_key():
+            return self.fetch_key()
+
+        # Is it the value indicator?
+        if ch == ':' and self.check_value():
+            return self.fetch_value()
+
+        # Is it an alias?
+        if ch == '*':
+            return self.fetch_alias()
+
+        # Is it an anchor?
+        if ch == '&':
+            return self.fetch_anchor()
+
+        # Is it a tag?
+        if ch == '!':
+            return self.fetch_tag()
+
+        # Is it a literal scalar?
+        if ch == '|' and not self.flow_level:
+            return self.fetch_literal()
+
+        # Is it a folded scalar?
+        if ch == '>' and not self.flow_level:
+            return self.fetch_folded()
+
+        # Is it a single quoted scalar?
+        if ch == "'":
+            return self.fetch_single()
+
+        # Is it a double quoted scalar?
+        if ch == '"':
+            return self.fetch_double()
+
+        # It must be a plain scalar then.
+        if self.check_plain():
+            return self.fetch_plain()
+
+        # No? It's an error. Let's produce a nice error message.
+        raise ScannerError(
+            'while scanning for the next token',
+            None,
+            'found character %r that cannot start any token' % utf8(ch),
+            self.reader.get_mark(),
+        )
+
+    # Simple keys treatment.
+
+    def next_possible_simple_key(self):
+        # type: () -> Any
+        # Return the number of the nearest possible simple key. Actually we
+        # don't need to loop through the whole dictionary. We may replace it
+        # with the following code:
+        #   if not self.possible_simple_keys:
+        #       return None
+        #   return self.possible_simple_keys[
+        #           min(self.possible_simple_keys.keys())].token_number
+        min_token_number = None
+        for level in self.possible_simple_keys:
+            key = self.possible_simple_keys[level]
+            if min_token_number is None or key.token_number < min_token_number:
+                min_token_number = key.token_number
+        return min_token_number
+
+    def stale_possible_simple_keys(self):
+        # type: () -> None
+        # Remove entries that are no longer possible simple keys. According to
+        # the YAML specification, simple keys
+        # - should be limited to a single line,
+        # - should be no longer than 1024 characters.
+        # Disabling this procedure will allow simple keys of any length and
+        # height (may cause problems if indentation is broken though).
+        for level in list(self.possible_simple_keys):
+            key = self.possible_simple_keys[level]
+            if key.line != self.reader.line or self.reader.index - key.index > 1024:
+                if key.required:
+                    raise ScannerError(
+                        'while scanning a simple key',
+                        key.mark,
+                        "could not find expected ':'",
+                        self.reader.get_mark(),
+                    )
+                del self.possible_simple_keys[level]
+
+    def save_possible_simple_key(self):
+        # type: () -> None
+        # The next token may start a simple key. We check if it's possible
+        # and save its position. This function is called for
+        #   ALIAS, ANCHOR, TAG, SCALAR(flow), '[', and '{'.
+
+        # Check if a simple key is required at the current position.
+        required = not self.flow_level and self.indent == self.reader.column
+
+        # The next token might be a simple key. Let's save it's number and
+        # position.
+        if self.allow_simple_key:
+            self.remove_possible_simple_key()
+            token_number = self.tokens_taken + len(self.tokens)
+            key = SimpleKey(
+                token_number,
+                required,
+                self.reader.index,
+                self.reader.line,
+                self.reader.column,
+                self.reader.get_mark(),
+            )
+            self.possible_simple_keys[self.flow_level] = key
+
+    def remove_possible_simple_key(self):
+        # type: () -> None
+        # Remove the saved possible key position at the current flow level.
+        if self.flow_level in self.possible_simple_keys:
+            key = self.possible_simple_keys[self.flow_level]
+
+            if key.required:
+                raise ScannerError(
+                    'while scanning a simple key',
+                    key.mark,
+                    "could not find expected ':'",
+                    self.reader.get_mark(),
+                )
+
+            del self.possible_simple_keys[self.flow_level]
+
+    # Indentation functions.
+
+    def unwind_indent(self, column):
+        # type: (Any) -> None
+        # In flow context, tokens should respect indentation.
+        # Actually the condition should be `self.indent >= column` according to
+        # the spec. But this condition will prohibit intuitively correct
+        # constructions such as
+        # key : {
+        # }
+        # ####
+        # if self.flow_level and self.indent > column:
+        #     raise ScannerError(None, None,
+        #             "invalid intendation or unclosed '[' or '{'",
+        #             self.reader.get_mark())
+
+        # In the flow context, indentation is ignored. We make the scanner less
+        # restrictive then specification requires.
+        if bool(self.flow_level):
+            return
+
+        # In block context, we may need to issue the BLOCK-END tokens.
+        while self.indent > column:
+            mark = self.reader.get_mark()
+            self.indent = self.indents.pop()
+            self.tokens.append(BlockEndToken(mark, mark))
+
+    def add_indent(self, column):
+        # type: (int) -> bool
+        # Check if we need to increase indentation.
+        if self.indent < column:
+            self.indents.append(self.indent)
+            self.indent = column
+            return True
+        return False
+
+    # Fetchers.
+
+    def fetch_stream_start(self):
+        # type: () -> None
+        # We always add STREAM-START as the first token and STREAM-END as the
+        # last token.
+        # Read the token.
+        mark = self.reader.get_mark()
+        # Add STREAM-START.
+        self.tokens.append(StreamStartToken(mark, mark, encoding=self.reader.encoding))
+
+    def fetch_stream_end(self):
+        # type: () -> None
+        # Set the current intendation to -1.
+        self.unwind_indent(-1)
+        # Reset simple keys.
+        self.remove_possible_simple_key()
+        self.allow_simple_key = False
+        self.possible_simple_keys = {}  # type: Dict[Any, Any]
+        # Read the token.
+        mark = self.reader.get_mark()
+        # Add STREAM-END.
+        self.tokens.append(StreamEndToken(mark, mark))
+        # The steam is finished.
+        self.done = True
+
+    def fetch_directive(self):
+        # type: () -> None
+        # Set the current intendation to -1.
+        self.unwind_indent(-1)
+
+        # Reset simple keys.
+        self.remove_possible_simple_key()
+        self.allow_simple_key = False
+
+        # Scan and add DIRECTIVE.
+        self.tokens.append(self.scan_directive())
+
+    def fetch_document_start(self):
+        # type: () -> None
+        self.fetch_document_indicator(DocumentStartToken)
+
+    def fetch_document_end(self):
+        # type: () -> None
+        self.fetch_document_indicator(DocumentEndToken)
+
+    def fetch_document_indicator(self, TokenClass):
+        # type: (Any) -> None
+        # Set the current intendation to -1.
+        self.unwind_indent(-1)
+
+        # Reset simple keys. Note that there could not be a block collection
+        # after '---'.
+        self.remove_possible_simple_key()
+        self.allow_simple_key = False
+
+        # Add DOCUMENT-START or DOCUMENT-END.
+        start_mark = self.reader.get_mark()
+        self.reader.forward(3)
+        end_mark = self.reader.get_mark()
+        self.tokens.append(TokenClass(start_mark, end_mark))
+
+    def fetch_flow_sequence_start(self):
+        # type: () -> None
+        self.fetch_flow_collection_start(FlowSequenceStartToken, to_push='[')
+
+    def fetch_flow_mapping_start(self):
+        # type: () -> None
+        self.fetch_flow_collection_start(FlowMappingStartToken, to_push='{')
+
+    def fetch_flow_collection_start(self, TokenClass, to_push):
+        # type: (Any, Text) -> None
+        # '[' and '{' may start a simple key.
+        self.save_possible_simple_key()
+        # Increase the flow level.
+        self.flow_context.append(to_push)
+        # Simple keys are allowed after '[' and '{'.
+        self.allow_simple_key = True
+        # Add FLOW-SEQUENCE-START or FLOW-MAPPING-START.
+        start_mark = self.reader.get_mark()
+        self.reader.forward()
+        end_mark = self.reader.get_mark()
+        self.tokens.append(TokenClass(start_mark, end_mark))
+
+    def fetch_flow_sequence_end(self):
+        # type: () -> None
+        self.fetch_flow_collection_end(FlowSequenceEndToken)
+
+    def fetch_flow_mapping_end(self):
+        # type: () -> None
+        self.fetch_flow_collection_end(FlowMappingEndToken)
+
+    def fetch_flow_collection_end(self, TokenClass):
+        # type: (Any) -> None
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+        # Decrease the flow level.
+        try:
+            popped = self.flow_context.pop()  # NOQA
+        except IndexError:
+            # We must not be in a list or object.
+            # Defer error handling to the parser.
+            pass
+        # No simple keys after ']' or '}'.
+        self.allow_simple_key = False
+        # Add FLOW-SEQUENCE-END or FLOW-MAPPING-END.
+        start_mark = self.reader.get_mark()
+        self.reader.forward()
+        end_mark = self.reader.get_mark()
+        self.tokens.append(TokenClass(start_mark, end_mark))
+
+    def fetch_flow_entry(self):
+        # type: () -> None
+        # Simple keys are allowed after ','.
+        self.allow_simple_key = True
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+        # Add FLOW-ENTRY.
+        start_mark = self.reader.get_mark()
+        self.reader.forward()
+        end_mark = self.reader.get_mark()
+        self.tokens.append(FlowEntryToken(start_mark, end_mark))
+
+    def fetch_block_entry(self):
+        # type: () -> None
+        # Block context needs additional checks.
+        if not self.flow_level:
+            # Are we allowed to start a new entry?
+            if not self.allow_simple_key:
+                raise ScannerError(
+                    None, None, 'sequence entries are not allowed here', self.reader.get_mark()
+                )
+            # We may need to add BLOCK-SEQUENCE-START.
+            if self.add_indent(self.reader.column):
+                mark = self.reader.get_mark()
+                self.tokens.append(BlockSequenceStartToken(mark, mark))
+        # It's an error for the block entry to occur in the flow context,
+        # but we let the parser detect this.
+        else:
+            pass
+        # Simple keys are allowed after '-'.
+        self.allow_simple_key = True
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+
+        # Add BLOCK-ENTRY.
+        start_mark = self.reader.get_mark()
+        self.reader.forward()
+        end_mark = self.reader.get_mark()
+        self.tokens.append(BlockEntryToken(start_mark, end_mark))
+
+    def fetch_key(self):
+        # type: () -> None
+        # Block context needs additional checks.
+        if not self.flow_level:
+
+            # Are we allowed to start a key (not nessesary a simple)?
+            if not self.allow_simple_key:
+                raise ScannerError(
+                    None, None, 'mapping keys are not allowed here', self.reader.get_mark()
+                )
+
+            # We may need to add BLOCK-MAPPING-START.
+            if self.add_indent(self.reader.column):
+                mark = self.reader.get_mark()
+                self.tokens.append(BlockMappingStartToken(mark, mark))
+
+        # Simple keys are allowed after '?' in the block context.
+        self.allow_simple_key = not self.flow_level
+
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+
+        # Add KEY.
+        start_mark = self.reader.get_mark()
+        self.reader.forward()
+        end_mark = self.reader.get_mark()
+        self.tokens.append(KeyToken(start_mark, end_mark))
+
+    def fetch_value(self):
+        # type: () -> None
+        # Do we determine a simple key?
+        if self.flow_level in self.possible_simple_keys:
+            # Add KEY.
+            key = self.possible_simple_keys[self.flow_level]
+            del self.possible_simple_keys[self.flow_level]
+            self.tokens.insert(
+                key.token_number - self.tokens_taken, KeyToken(key.mark, key.mark)
+            )
+
+            # If this key starts a new block mapping, we need to add
+            # BLOCK-MAPPING-START.
+            if not self.flow_level:
+                if self.add_indent(key.column):
+                    self.tokens.insert(
+                        key.token_number - self.tokens_taken,
+                        BlockMappingStartToken(key.mark, key.mark),
+                    )
+
+            # There cannot be two simple keys one after another.
+            self.allow_simple_key = False
+
+        # It must be a part of a complex key.
+        else:
+
+            # Block context needs additional checks.
+            # (Do we really need them? They will be caught by the parser
+            # anyway.)
+            if not self.flow_level:
+
+                # We are allowed to start a complex value if and only if
+                # we can start a simple key.
+                if not self.allow_simple_key:
+                    raise ScannerError(
+                        None,
+                        None,
+                        'mapping values are not allowed here',
+                        self.reader.get_mark(),
+                    )
+
+            # If this value starts a new block mapping, we need to add
+            # BLOCK-MAPPING-START.  It will be detected as an error later by
+            # the parser.
+            if not self.flow_level:
+                if self.add_indent(self.reader.column):
+                    mark = self.reader.get_mark()
+                    self.tokens.append(BlockMappingStartToken(mark, mark))
+
+            # Simple keys are allowed after ':' in the block context.
+            self.allow_simple_key = not self.flow_level
+
+            # Reset possible simple key on the current level.
+            self.remove_possible_simple_key()
+
+        # Add VALUE.
+        start_mark = self.reader.get_mark()
+        self.reader.forward()
+        end_mark = self.reader.get_mark()
+        self.tokens.append(ValueToken(start_mark, end_mark))
+
+    def fetch_alias(self):
+        # type: () -> None
+        # ALIAS could be a simple key.
+        self.save_possible_simple_key()
+        # No simple keys after ALIAS.
+        self.allow_simple_key = False
+        # Scan and add ALIAS.
+        self.tokens.append(self.scan_anchor(AliasToken))
+
+    def fetch_anchor(self):
+        # type: () -> None
+        # ANCHOR could start a simple key.
+        self.save_possible_simple_key()
+        # No simple keys after ANCHOR.
+        self.allow_simple_key = False
+        # Scan and add ANCHOR.
+        self.tokens.append(self.scan_anchor(AnchorToken))
+
+    def fetch_tag(self):
+        # type: () -> None
+        # TAG could start a simple key.
+        self.save_possible_simple_key()
+        # No simple keys after TAG.
+        self.allow_simple_key = False
+        # Scan and add TAG.
+        self.tokens.append(self.scan_tag())
+
+    def fetch_literal(self):
+        # type: () -> None
+        self.fetch_block_scalar(style='|')
+
+    def fetch_folded(self):
+        # type: () -> None
+        self.fetch_block_scalar(style='>')
+
+    def fetch_block_scalar(self, style):
+        # type: (Any) -> None
+        # A simple key may follow a block scalar.
+        self.allow_simple_key = True
+        # Reset possible simple key on the current level.
+        self.remove_possible_simple_key()
+        # Scan and add SCALAR.
+        self.tokens.append(self.scan_block_scalar(style))
+
+    def fetch_single(self):
+        # type: () -> None
+        self.fetch_flow_scalar(style="'")
+
+    def fetch_double(self):
+        # type: () -> None
+        self.fetch_flow_scalar(style='"')
+
+    def fetch_flow_scalar(self, style):
+        # type: (Any) -> None
+        # A flow scalar could be a simple key.
+        self.save_possible_simple_key()
+        # No simple keys after flow scalars.
+        self.allow_simple_key = False
+        # Scan and add SCALAR.
+        self.tokens.append(self.scan_flow_scalar(style))
+
+    def fetch_plain(self):
+        # type: () -> None
+        # A plain scalar could be a simple key.
+        self.save_possible_simple_key()
+        # No simple keys after plain scalars. But note that `scan_plain` will
+        # change this flag if the scan is finished at the beginning of the
+        # line.
+        self.allow_simple_key = False
+        # Scan and add SCALAR. May change `allow_simple_key`.
+        self.tokens.append(self.scan_plain())
+
+    # Checkers.
+
+    def check_directive(self):
+        # type: () -> Any
+        # DIRECTIVE:        ^ '%' ...
+        # The '%' indicator is already checked.
+        if self.reader.column == 0:
+            return True
+        return None
+
+    def check_document_start(self):
+        # type: () -> Any
+        # DOCUMENT-START:   ^ '---' (' '|'\n')
+        if self.reader.column == 0:
+            if self.reader.prefix(3) == '---' and self.reader.peek(3) in _THE_END_SPACE_TAB:
+                return True
+        return None
+
+    def check_document_end(self):
+        # type: () -> Any
+        # DOCUMENT-END:     ^ '...' (' '|'\n')
+        if self.reader.column == 0:
+            if self.reader.prefix(3) == '...' and self.reader.peek(3) in _THE_END_SPACE_TAB:
+                return True
+        return None
+
+    def check_block_entry(self):
+        # type: () -> Any
+        # BLOCK-ENTRY:      '-' (' '|'\n')
+        return self.reader.peek(1) in _THE_END_SPACE_TAB
+
+    def check_key(self):
+        # type: () -> Any
+        # KEY(flow context):    '?'
+        if bool(self.flow_level):
+            return True
+        # KEY(block context):   '?' (' '|'\n')
+        return self.reader.peek(1) in _THE_END_SPACE_TAB
+
+    def check_value(self):
+        # type: () -> Any
+        # VALUE(flow context):  ':'
+        if self.scanner_processing_version == (1, 1):
+            if bool(self.flow_level):
+                return True
+        else:
+            if bool(self.flow_level):
+                if self.flow_context[-1] == '[':
+                    if self.reader.peek(1) not in _THE_END_SPACE_TAB:
+                        return False
+                elif self.tokens and isinstance(self.tokens[-1], ValueToken):
+                    # mapping flow context scanning a value token
+                    if self.reader.peek(1) not in _THE_END_SPACE_TAB:
+                        return False
+                return True
+        # VALUE(block context): ':' (' '|'\n')
+        return self.reader.peek(1) in _THE_END_SPACE_TAB
+
+    def check_plain(self):
+        # type: () -> Any
+        # A plain scalar may start with any non-space character except:
+        #   '-', '?', ':', ',', '[', ']', '{', '}',
+        #   '#', '&', '*', '!', '|', '>', '\'', '\"',
+        #   '%', '@', '`'.
+        #
+        # It may also start with
+        #   '-', '?', ':'
+        # if it is followed by a non-space character.
+        #
+        # Note that we limit the last rule to the block context (except the
+        # '-' character) because we want the flow context to be space
+        # independent.
+        srp = self.reader.peek
+        ch = srp()
+        if self.scanner_processing_version == (1, 1):
+            return ch not in '\0 \t\r\n\x85\u2028\u2029-?:,[]{}#&*!|>\'"%@`' or (
+                srp(1) not in _THE_END_SPACE_TAB
+                and (ch == '-' or (not self.flow_level and ch in '?:'))
+            )
+        # YAML 1.2
+        if ch not in '\0 \t\r\n\x85\u2028\u2029-?:,[]{}#&*!|>\'"%@`':
+            # ###################                ^ ???
+            return True
+        ch1 = srp(1)
+        if ch == '-' and ch1 not in _THE_END_SPACE_TAB:
+            return True
+        if ch == ':' and bool(self.flow_level) and ch1 not in _SPACE_TAB:
+            return True
+
+        return srp(1) not in _THE_END_SPACE_TAB and (
+            ch == '-' or (not self.flow_level and ch in '?:')
+        )
+
+    # Scanners.
+
+    def scan_to_next_token(self):
+        # type: () -> Any
+        # We ignore spaces, line breaks and comments.
+        # If we find a line break in the block context, we set the flag
+        # `allow_simple_key` on.
+        # The byte order mark is stripped if it's the first character in the
+        # stream. We do not yet support BOM inside the stream as the
+        # specification requires. Any such mark will be considered as a part
+        # of the document.
+        #
+        # TODO: We need to make tab handling rules more sane. A good rule is
+        #   Tabs cannot precede tokens
+        #   BLOCK-SEQUENCE-START, BLOCK-MAPPING-START, BLOCK-END,
+        #   KEY(block), VALUE(block), BLOCK-ENTRY
+        # So the checking code is
+        #   if <TAB>:
+        #       self.allow_simple_keys = False
+        # We also need to add the check for `allow_simple_keys == True` to
+        # `unwind_indent` before issuing BLOCK-END.
+        # Scanners for block, flow, and plain scalars need to be modified.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        if self.reader.index == 0 and srp() == '\uFEFF':
+            srf()
+        found = False
+        _the_end = _THE_END
+        while not found:
+            while srp() == ' ':
+                srf()
+            if srp() == '#':
+                while srp() not in _the_end:
+                    srf()
+            if self.scan_line_break():
+                if not self.flow_level:
+                    self.allow_simple_key = True
+            else:
+                found = True
+        return None
+
+    def scan_directive(self):
+        # type: () -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        start_mark = self.reader.get_mark()
+        srf()
+        name = self.scan_directive_name(start_mark)
+        value = None
+        if name == 'YAML':
+            value = self.scan_yaml_directive_value(start_mark)
+            end_mark = self.reader.get_mark()
+        elif name == 'TAG':
+            value = self.scan_tag_directive_value(start_mark)
+            end_mark = self.reader.get_mark()
+        else:
+            end_mark = self.reader.get_mark()
+            while srp() not in _THE_END:
+                srf()
+        self.scan_directive_ignored_line(start_mark)
+        return DirectiveToken(name, value, start_mark, end_mark)
+
+    def scan_directive_name(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        length = 0
+        srp = self.reader.peek
+        ch = srp(length)
+        while '0' <= ch <= '9' or 'A' <= ch <= 'Z' or 'a' <= ch <= 'z' or ch in '-_:.':
+            length += 1
+            ch = srp(length)
+        if not length:
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                'expected alphabetic or numeric character, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        value = self.reader.prefix(length)
+        self.reader.forward(length)
+        ch = srp()
+        if ch not in '\0 \r\n\x85\u2028\u2029':
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                'expected alphabetic or numeric character, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        return value
+
+    def scan_yaml_directive_value(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        while srp() == ' ':
+            srf()
+        major = self.scan_yaml_directive_number(start_mark)
+        if srp() != '.':
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                "expected a digit or '.', but found %r" % utf8(srp()),
+                self.reader.get_mark(),
+            )
+        srf()
+        minor = self.scan_yaml_directive_number(start_mark)
+        if srp() not in '\0 \r\n\x85\u2028\u2029':
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                "expected a digit or ' ', but found %r" % utf8(srp()),
+                self.reader.get_mark(),
+            )
+        return (major, minor)
+
+    def scan_yaml_directive_number(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        ch = srp()
+        if not ('0' <= ch <= '9'):
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                'expected a digit, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        length = 0
+        while '0' <= srp(length) <= '9':
+            length += 1
+        value = int(self.reader.prefix(length))
+        srf(length)
+        return value
+
+    def scan_tag_directive_value(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        while srp() == ' ':
+            srf()
+        handle = self.scan_tag_directive_handle(start_mark)
+        while srp() == ' ':
+            srf()
+        prefix = self.scan_tag_directive_prefix(start_mark)
+        return (handle, prefix)
+
+    def scan_tag_directive_handle(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        value = self.scan_tag_handle('directive', start_mark)
+        ch = self.reader.peek()
+        if ch != ' ':
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                "expected ' ', but found %r" % utf8(ch),
+                self.reader.get_mark(),
+            )
+        return value
+
+    def scan_tag_directive_prefix(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        value = self.scan_tag_uri('directive', start_mark)
+        ch = self.reader.peek()
+        if ch not in '\0 \r\n\x85\u2028\u2029':
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                "expected ' ', but found %r" % utf8(ch),
+                self.reader.get_mark(),
+            )
+        return value
+
+    def scan_directive_ignored_line(self, start_mark):
+        # type: (Any) -> None
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        while srp() == ' ':
+            srf()
+        if srp() == '#':
+            while srp() not in _THE_END:
+                srf()
+        ch = srp()
+        if ch not in _THE_END:
+            raise ScannerError(
+                'while scanning a directive',
+                start_mark,
+                'expected a comment or a line break, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        self.scan_line_break()
+
+    def scan_anchor(self, TokenClass):
+        # type: (Any) -> Any
+        # The specification does not restrict characters for anchors and
+        # aliases. This may lead to problems, for instance, the document:
+        #   [ *alias, value ]
+        # can be interpteted in two ways, as
+        #   [ "value" ]
+        # and
+        #   [ *alias , "value" ]
+        # Therefore we restrict aliases to numbers and ASCII letters.
+        srp = self.reader.peek
+        start_mark = self.reader.get_mark()
+        indicator = srp()
+        if indicator == '*':
+            name = 'alias'
+        else:
+            name = 'anchor'
+        self.reader.forward()
+        length = 0
+        ch = srp(length)
+        # while u'0' <= ch <= u'9' or u'A' <= ch <= u'Z' or u'a' <= ch <= u'z' \
+        #         or ch in u'-_':
+        while check_anchorname_char(ch):
+            length += 1
+            ch = srp(length)
+        if not length:
+            raise ScannerError(
+                'while scanning an %s' % (name,),
+                start_mark,
+                'expected alphabetic or numeric character, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        value = self.reader.prefix(length)
+        self.reader.forward(length)
+        # ch1 = ch
+        # ch = srp()   # no need to peek, ch is already set
+        # assert ch1 == ch
+        if ch not in '\0 \t\r\n\x85\u2028\u2029?:,[]{}%@`':
+            raise ScannerError(
+                'while scanning an %s' % (name,),
+                start_mark,
+                'expected alphabetic or numeric character, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        end_mark = self.reader.get_mark()
+        return TokenClass(value, start_mark, end_mark)
+
+    def scan_tag(self):
+        # type: () -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        start_mark = self.reader.get_mark()
+        ch = srp(1)
+        if ch == '<':
+            handle = None
+            self.reader.forward(2)
+            suffix = self.scan_tag_uri('tag', start_mark)
+            if srp() != '>':
+                raise ScannerError(
+                    'while parsing a tag',
+                    start_mark,
+                    "expected '>', but found %r" % utf8(srp()),
+                    self.reader.get_mark(),
+                )
+            self.reader.forward()
+        elif ch in _THE_END_SPACE_TAB:
+            handle = None
+            suffix = '!'
+            self.reader.forward()
+        else:
+            length = 1
+            use_handle = False
+            while ch not in '\0 \r\n\x85\u2028\u2029':
+                if ch == '!':
+                    use_handle = True
+                    break
+                length += 1
+                ch = srp(length)
+            handle = '!'
+            if use_handle:
+                handle = self.scan_tag_handle('tag', start_mark)
+            else:
+                handle = '!'
+                self.reader.forward()
+            suffix = self.scan_tag_uri('tag', start_mark)
+        ch = srp()
+        if ch not in '\0 \r\n\x85\u2028\u2029':
+            raise ScannerError(
+                'while scanning a tag',
+                start_mark,
+                "expected ' ', but found %r" % utf8(ch),
+                self.reader.get_mark(),
+            )
+        value = (handle, suffix)
+        end_mark = self.reader.get_mark()
+        return TagToken(value, start_mark, end_mark)
+
+    def scan_block_scalar(self, style, rt=False):
+        # type: (Any, Optional[bool]) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        if style == '>':
+            folded = True
+        else:
+            folded = False
+
+        chunks = []  # type: List[Any]
+        start_mark = self.reader.get_mark()
+
+        # Scan the header.
+        self.reader.forward()
+        chomping, increment = self.scan_block_scalar_indicators(start_mark)
+        # block scalar comment e.g. : |+  # comment text
+        block_scalar_comment = self.scan_block_scalar_ignored_line(start_mark)
+
+        # Determine the indentation level and go to the first non-empty line.
+        min_indent = self.indent + 1
+        if increment is None:
+            # no increment and top level, min_indent could be 0
+            if min_indent < 1 and (
+                style not in '|>'
+                or (self.scanner_processing_version == (1, 1))
+                and getattr(
+                    self.loader, 'top_level_block_style_scalar_no_indent_error_1_1', False
+                )
+            ):
+                min_indent = 1
+            breaks, max_indent, end_mark = self.scan_block_scalar_indentation()
+            indent = max(min_indent, max_indent)
+        else:
+            if min_indent < 1:
+                min_indent = 1
+            indent = min_indent + increment - 1
+            breaks, end_mark = self.scan_block_scalar_breaks(indent)
+        line_break = ""
+
+        # Scan the inner part of the block scalar.
+        while self.reader.column == indent and srp() != '\0':
+            chunks.extend(breaks)
+            leading_non_space = srp() not in ' \t'
+            length = 0
+            while srp(length) not in _THE_END:
+                length += 1
+            chunks.append(self.reader.prefix(length))
+            self.reader.forward(length)
+            line_break = self.scan_line_break()
+            breaks, end_mark = self.scan_block_scalar_breaks(indent)
+            if style in '|>' and min_indent == 0:
+                # at the beginning of a line, if in block style see if
+                # end of document/start_new_document
+                if self.check_document_start() or self.check_document_end():
+                    break
+            if self.reader.column == indent and srp() != '\0':
+
+                # Unfortunately, folding rules are ambiguous.
+                #
+                # This is the folding according to the specification:
+
+                if rt and folded and line_break == '\n':
+                    chunks.append('\a')
+                if folded and line_break == '\n' and leading_non_space and srp() not in ' \t':
+                    if not breaks:
+                        chunks.append(' ')
+                else:
+                    chunks.append(line_break)
+
+                # This is Clark Evans's interpretation (also in the spec
+                # examples):
+                #
+                # if folded and line_break == u'\n':
+                #     if not breaks:
+                #         if srp() not in ' \t':
+                #             chunks.append(u' ')
+                #         else:
+                #             chunks.append(line_break)
+                # else:
+                #     chunks.append(line_break)
+            else:
+                break
+
+        # Process trailing line breaks. The 'chomping' setting determines
+        # whether they are included in the value.
+        trailing = []  # type: List[Any]
+        if chomping in [None, True]:
+            chunks.append(line_break)
+        if chomping is True:
+            chunks.extend(breaks)
+        elif chomping in [None, False]:
+            trailing.extend(breaks)
+
+        # We are done.
+        token = ScalarToken("".join(chunks), False, start_mark, end_mark, style)
+        if block_scalar_comment is not None:
+            token.add_pre_comments([block_scalar_comment])
+        if len(trailing) > 0:
+            # nprint('trailing 1', trailing)  # XXXXX
+            # Eat whitespaces and comments until we reach the next token.
+            comment = self.scan_to_next_token()
+            while comment:
+                trailing.append(' ' * comment[1].column + comment[0])
+                comment = self.scan_to_next_token()
+
+            # Keep track of the trailing whitespace and following comments
+            # as a comment token, if isn't all included in the actual value.
+            comment_end_mark = self.reader.get_mark()
+            comment = CommentToken("".join(trailing), end_mark, comment_end_mark)
+            token.add_post_comment(comment)
+        return token
+
+    def scan_block_scalar_indicators(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        chomping = None
+        increment = None
+        ch = srp()
+        if ch in '+-':
+            if ch == '+':
+                chomping = True
+            else:
+                chomping = False
+            self.reader.forward()
+            ch = srp()
+            if ch in '0123456789':
+                increment = int(ch)
+                if increment == 0:
+                    raise ScannerError(
+                        'while scanning a block scalar',
+                        start_mark,
+                        'expected indentation indicator in the range 1-9, ' 'but found 0',
+                        self.reader.get_mark(),
+                    )
+                self.reader.forward()
+        elif ch in '0123456789':
+            increment = int(ch)
+            if increment == 0:
+                raise ScannerError(
+                    'while scanning a block scalar',
+                    start_mark,
+                    'expected indentation indicator in the range 1-9, ' 'but found 0',
+                    self.reader.get_mark(),
+                )
+            self.reader.forward()
+            ch = srp()
+            if ch in '+-':
+                if ch == '+':
+                    chomping = True
+                else:
+                    chomping = False
+                self.reader.forward()
+        ch = srp()
+        if ch not in '\0 \r\n\x85\u2028\u2029':
+            raise ScannerError(
+                'while scanning a block scalar',
+                start_mark,
+                'expected chomping or indentation indicators, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        return chomping, increment
+
+    def scan_block_scalar_ignored_line(self, start_mark):
+        # type: (Any) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        prefix = ''
+        comment = None
+        while srp() == ' ':
+            prefix += srp()
+            srf()
+        if srp() == '#':
+            comment = prefix
+            while srp() not in _THE_END:
+                comment += srp()
+                srf()
+        ch = srp()
+        if ch not in _THE_END:
+            raise ScannerError(
+                'while scanning a block scalar',
+                start_mark,
+                'expected a comment or a line break, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        self.scan_line_break()
+        return comment
+
+    def scan_block_scalar_indentation(self):
+        # type: () -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        chunks = []
+        max_indent = 0
+        end_mark = self.reader.get_mark()
+        while srp() in ' \r\n\x85\u2028\u2029':
+            if srp() != ' ':
+                chunks.append(self.scan_line_break())
+                end_mark = self.reader.get_mark()
+            else:
+                srf()
+                if self.reader.column > max_indent:
+                    max_indent = self.reader.column
+        return chunks, max_indent, end_mark
+
+    def scan_block_scalar_breaks(self, indent):
+        # type: (int) -> Any
+        # See the specification for details.
+        chunks = []
+        srp = self.reader.peek
+        srf = self.reader.forward
+        end_mark = self.reader.get_mark()
+        while self.reader.column < indent and srp() == ' ':
+            srf()
+        while srp() in '\r\n\x85\u2028\u2029':
+            chunks.append(self.scan_line_break())
+            end_mark = self.reader.get_mark()
+            while self.reader.column < indent and srp() == ' ':
+                srf()
+        return chunks, end_mark
+
+    def scan_flow_scalar(self, style):
+        # type: (Any) -> Any
+        # See the specification for details.
+        # Note that we loose indentation rules for quoted scalars. Quoted
+        # scalars don't need to adhere indentation because " and ' clearly
+        # mark the beginning and the end of them. Therefore we are less
+        # restrictive then the specification requires. We only need to check
+        # that document separators are not included in scalars.
+        if style == '"':
+            double = True
+        else:
+            double = False
+        srp = self.reader.peek
+        chunks = []  # type: List[Any]
+        start_mark = self.reader.get_mark()
+        quote = srp()
+        self.reader.forward()
+        chunks.extend(self.scan_flow_scalar_non_spaces(double, start_mark))
+        while srp() != quote:
+            chunks.extend(self.scan_flow_scalar_spaces(double, start_mark))
+            chunks.extend(self.scan_flow_scalar_non_spaces(double, start_mark))
+        self.reader.forward()
+        end_mark = self.reader.get_mark()
+        return ScalarToken("".join(chunks), False, start_mark, end_mark, style)
+
+    ESCAPE_REPLACEMENTS = {
+        '0': '\0',
+        'a': '\x07',
+        'b': '\x08',
+        't': '\x09',
+        '\t': '\x09',
+        'n': '\x0A',
+        'v': '\x0B',
+        'f': '\x0C',
+        'r': '\x0D',
+        'e': '\x1B',
+        ' ': '\x20',
+        '"': '"',
+        '/': '/',  # as per http://www.json.org/
+        '\\': '\\',
+        'N': '\x85',
+        '_': '\xA0',
+        'L': '\u2028',
+        'P': '\u2029',
+    }
+
+    ESCAPE_CODES = {'x': 2, 'u': 4, 'U': 8}
+
+    def scan_flow_scalar_non_spaces(self, double, start_mark):
+        # type: (Any, Any) -> Any
+        # See the specification for details.
+        chunks = []  # type: List[Any]
+        srp = self.reader.peek
+        srf = self.reader.forward
+        while True:
+            length = 0
+            while srp(length) not in ' \n\'"\\\0\t\r\x85\u2028\u2029':
+                length += 1
+            if length != 0:
+                chunks.append(self.reader.prefix(length))
+                srf(length)
+            ch = srp()
+            if not double and ch == "'" and srp(1) == "'":
+                chunks.append("'")
+                srf(2)
+            elif (double and ch == "'") or (not double and ch in '"\\'):
+                chunks.append(ch)
+                srf()
+            elif double and ch == '\\':
+                srf()
+                ch = srp()
+                if ch in self.ESCAPE_REPLACEMENTS:
+                    chunks.append(self.ESCAPE_REPLACEMENTS[ch])
+                    srf()
+                elif ch in self.ESCAPE_CODES:
+                    length = self.ESCAPE_CODES[ch]
+                    srf()
+                    for k in range(length):
+                        if srp(k) not in '0123456789ABCDEFabcdef':
+                            raise ScannerError(
+                                'while scanning a double-quoted scalar',
+                                start_mark,
+                                'expected escape sequence of %d hexdecimal '
+                                'numbers, but found %r' % (length, utf8(srp(k))),
+                                self.reader.get_mark(),
+                            )
+                    code = int(self.reader.prefix(length), 16)
+                    chunks.append(unichr(code))
+                    srf(length)
+                elif ch in '\n\r\x85\u2028\u2029':
+                    self.scan_line_break()
+                    chunks.extend(self.scan_flow_scalar_breaks(double, start_mark))
+                else:
+                    raise ScannerError(
+                        'while scanning a double-quoted scalar',
+                        start_mark,
+                        'found unknown escape character %r' % utf8(ch),
+                        self.reader.get_mark(),
+                    )
+            else:
+                return chunks
+
+    def scan_flow_scalar_spaces(self, double, start_mark):
+        # type: (Any, Any) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        chunks = []
+        length = 0
+        while srp(length) in ' \t':
+            length += 1
+        whitespaces = self.reader.prefix(length)
+        self.reader.forward(length)
+        ch = srp()
+        if ch == '\0':
+            raise ScannerError(
+                'while scanning a quoted scalar',
+                start_mark,
+                'found unexpected end of stream',
+                self.reader.get_mark(),
+            )
+        elif ch in '\r\n\x85\u2028\u2029':
+            line_break = self.scan_line_break()
+            breaks = self.scan_flow_scalar_breaks(double, start_mark)
+            if line_break != '\n':
+                chunks.append(line_break)
+            elif not breaks:
+                chunks.append(' ')
+            chunks.extend(breaks)
+        else:
+            chunks.append(whitespaces)
+        return chunks
+
+    def scan_flow_scalar_breaks(self, double, start_mark):
+        # type: (Any, Any) -> Any
+        # See the specification for details.
+        chunks = []  # type: List[Any]
+        srp = self.reader.peek
+        srf = self.reader.forward
+        while True:
+            # Instead of checking indentation, we check for document
+            # separators.
+            prefix = self.reader.prefix(3)
+            if (prefix == '---' or prefix == '...') and srp(3) in _THE_END_SPACE_TAB:
+                raise ScannerError(
+                    'while scanning a quoted scalar',
+                    start_mark,
+                    'found unexpected document separator',
+                    self.reader.get_mark(),
+                )
+            while srp() in ' \t':
+                srf()
+            if srp() in '\r\n\x85\u2028\u2029':
+                chunks.append(self.scan_line_break())
+            else:
+                return chunks
+
+    def scan_plain(self):
+        # type: () -> Any
+        # See the specification for details.
+        # We add an additional restriction for the flow context:
+        #   plain scalars in the flow context cannot contain ',', ': '  and '?'.
+        # We also keep track of the `allow_simple_key` flag here.
+        # Indentation rules are loosed for the flow context.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        chunks = []  # type: List[Any]
+        start_mark = self.reader.get_mark()
+        end_mark = start_mark
+        indent = self.indent + 1
+        # We allow zero indentation for scalars, but then we need to check for
+        # document separators at the beginning of the line.
+        # if indent == 0:
+        #     indent = 1
+        spaces = []  # type: List[Any]
+        while True:
+            length = 0
+            if srp() == '#':
+                break
+            while True:
+                ch = srp(length)
+                if ch == ':' and srp(length + 1) not in _THE_END_SPACE_TAB:
+                    pass
+                elif ch == '?' and self.scanner_processing_version != (1, 1):
+                    pass
+                elif (
+                    ch in _THE_END_SPACE_TAB
+                    or (
+                        not self.flow_level
+                        and ch == ':'
+                        and srp(length + 1) in _THE_END_SPACE_TAB
+                    )
+                    or (self.flow_level and ch in ',:?[]{}')
+                ):
+                    break
+                length += 1
+            # It's not clear what we should do with ':' in the flow context.
+            if (
+                self.flow_level
+                and ch == ':'
+                and srp(length + 1) not in '\0 \t\r\n\x85\u2028\u2029,[]{}'
+            ):
+                srf(length)
+                raise ScannerError(
+                    'while scanning a plain scalar',
+                    start_mark,
+                    "found unexpected ':'",
+                    self.reader.get_mark(),
+                    'Please check '
+                    'http://pyyaml.org/wiki/YAMLColonInFlowContext '
+                    'for details.',
+                )
+            if length == 0:
+                break
+            self.allow_simple_key = False
+            chunks.extend(spaces)
+            chunks.append(self.reader.prefix(length))
+            srf(length)
+            end_mark = self.reader.get_mark()
+            spaces = self.scan_plain_spaces(indent, start_mark)
+            if (
+                not spaces
+                or srp() == '#'
+                or (not self.flow_level and self.reader.column < indent)
+            ):
+                break
+
+        token = ScalarToken("".join(chunks), True, start_mark, end_mark)
+        if spaces and spaces[0] == '\n':
+            # Create a comment token to preserve the trailing line breaks.
+            comment = CommentToken("".join(spaces) + '\n', start_mark, end_mark)
+            token.add_post_comment(comment)
+        return token
+
+    def scan_plain_spaces(self, indent, start_mark):
+        # type: (Any, Any) -> Any
+        # See the specification for details.
+        # The specification is really confusing about tabs in plain scalars.
+        # We just forbid them completely. Do not use tabs in YAML!
+        srp = self.reader.peek
+        srf = self.reader.forward
+        chunks = []
+        length = 0
+        while srp(length) in ' ':
+            length += 1
+        whitespaces = self.reader.prefix(length)
+        self.reader.forward(length)
+        ch = srp()
+        if ch in '\r\n\x85\u2028\u2029':
+            line_break = self.scan_line_break()
+            self.allow_simple_key = True
+            prefix = self.reader.prefix(3)
+            if (prefix == '---' or prefix == '...') and srp(3) in _THE_END_SPACE_TAB:
+                return
+            breaks = []
+            while srp() in ' \r\n\x85\u2028\u2029':
+                if srp() == ' ':
+                    srf()
+                else:
+                    breaks.append(self.scan_line_break())
+                    prefix = self.reader.prefix(3)
+                    if (prefix == '---' or prefix == '...') and srp(3) in _THE_END_SPACE_TAB:
+                        return
+            if line_break != '\n':
+                chunks.append(line_break)
+            elif not breaks:
+                chunks.append(' ')
+            chunks.extend(breaks)
+        elif whitespaces:
+            chunks.append(whitespaces)
+        return chunks
+
+    def scan_tag_handle(self, name, start_mark):
+        # type: (Any, Any) -> Any
+        # See the specification for details.
+        # For some strange reasons, the specification does not allow '_' in
+        # tag handles. I have allowed it anyway.
+        srp = self.reader.peek
+        ch = srp()
+        if ch != '!':
+            raise ScannerError(
+                'while scanning a %s' % (name,),
+                start_mark,
+                "expected '!', but found %r" % utf8(ch),
+                self.reader.get_mark(),
+            )
+        length = 1
+        ch = srp(length)
+        if ch != ' ':
+            while '0' <= ch <= '9' or 'A' <= ch <= 'Z' or 'a' <= ch <= 'z' or ch in '-_':
+                length += 1
+                ch = srp(length)
+            if ch != '!':
+                self.reader.forward(length)
+                raise ScannerError(
+                    'while scanning a %s' % (name,),
+                    start_mark,
+                    "expected '!', but found %r" % utf8(ch),
+                    self.reader.get_mark(),
+                )
+            length += 1
+        value = self.reader.prefix(length)
+        self.reader.forward(length)
+        return value
+
+    def scan_tag_uri(self, name, start_mark):
+        # type: (Any, Any) -> Any
+        # See the specification for details.
+        # Note: we do not check if URI is well-formed.
+        srp = self.reader.peek
+        chunks = []
+        length = 0
+        ch = srp(length)
+        while (
+            '0' <= ch <= '9'
+            or 'A' <= ch <= 'Z'
+            or 'a' <= ch <= 'z'
+            or ch in "-;/?:@&=+$,_.!~*'()[]%"
+        ):
+            if ch == '%':
+                chunks.append(self.reader.prefix(length))
+                self.reader.forward(length)
+                length = 0
+                chunks.append(self.scan_uri_escapes(name, start_mark))
+            else:
+                length += 1
+            ch = srp(length)
+        if length != 0:
+            chunks.append(self.reader.prefix(length))
+            self.reader.forward(length)
+            length = 0
+        if not chunks:
+            raise ScannerError(
+                'while parsing a %s' % (name,),
+                start_mark,
+                'expected URI, but found %r' % utf8(ch),
+                self.reader.get_mark(),
+            )
+        return "".join(chunks)
+
+    def scan_uri_escapes(self, name, start_mark):
+        # type: (Any, Any) -> Any
+        # See the specification for details.
+        srp = self.reader.peek
+        srf = self.reader.forward
+        code_bytes = []  # type: List[Any]
+        mark = self.reader.get_mark()
+        while srp() == '%':
+            srf()
+            for k in range(2):
+                if srp(k) not in '0123456789ABCDEFabcdef':
+                    raise ScannerError(
+                        'while scanning a %s' % (name,),
+                        start_mark,
+                        'expected URI escape sequence of 2 hexdecimal numbers,'
+                        ' but found %r' % utf8(srp(k)),
+                        self.reader.get_mark(),
+                    )
+            if PY3:
+                code_bytes.append(int(self.reader.prefix(2), 16))
+            else:
+                code_bytes.append(chr(int(self.reader.prefix(2), 16)))
+            srf(2)
+        try:
+            if PY3:
+                value = bytes(code_bytes).decode('utf-8')
+            else:
+                value = unicode(b"".join(code_bytes), 'utf-8')
+        except UnicodeDecodeError as exc:
+            raise ScannerError('while scanning a %s' % (name,), start_mark, str(exc), mark)
+        return value
+
+    def scan_line_break(self):
+        # type: () -> Any
+        # Transforms:
+        #   '\r\n'      :   '\n'
+        #   '\r'        :   '\n'
+        #   '\n'        :   '\n'
+        #   '\x85'      :   '\n'
+        #   '\u2028'    :   '\u2028'
+        #   '\u2029     :   '\u2029'
+        #   default     :   ''
+        ch = self.reader.peek()
+        if ch in '\r\n\x85':
+            if self.reader.prefix(2) == '\r\n':
+                self.reader.forward(2)
+            else:
+                self.reader.forward()
+            return '\n'
+        elif ch in '\u2028\u2029':
+            self.reader.forward()
+            return ch
+        return ""
+
+
+class RoundTripScanner(Scanner):
+    def check_token(self, *choices):
+        # type: (Any) -> bool
+        # Check if the next token is one of the given types.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        self._gather_comments()
+        if bool(self.tokens):
+            if not choices:
+                return True
+            for choice in choices:
+                if isinstance(self.tokens[0], choice):
+                    return True
+        return False
+
+    def peek_token(self):
+        # type: () -> Any
+        # Return the next token, but do not delete if from the queue.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        self._gather_comments()
+        if bool(self.tokens):
+            return self.tokens[0]
+        return None
+
+    def _gather_comments(self):
+        # type: () -> Any
+        """combine multiple comment lines"""
+        comments = []  # type: List[Any]
+        if not self.tokens:
+            return comments
+        if isinstance(self.tokens[0], CommentToken):
+            comment = self.tokens.pop(0)
+            self.tokens_taken += 1
+            comments.append(comment)
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+            if not self.tokens:
+                return comments
+            if isinstance(self.tokens[0], CommentToken):
+                self.tokens_taken += 1
+                comment = self.tokens.pop(0)
+                # nprint('dropping2', comment)
+                comments.append(comment)
+        if len(comments) >= 1:
+            self.tokens[0].add_pre_comments(comments)
+        # pull in post comment on e.g. ':'
+        if not self.done and len(self.tokens) < 2:
+            self.fetch_more_tokens()
+
+    def get_token(self):
+        # type: () -> Any
+        # Return the next token.
+        while self.need_more_tokens():
+            self.fetch_more_tokens()
+        self._gather_comments()
+        if bool(self.tokens):
+            # nprint('tk', self.tokens)
+            # only add post comment to single line tokens:
+            # scalar, value token. FlowXEndToken, otherwise
+            # hidden streamtokens could get them (leave them and they will be
+            # pre comments for the next map/seq
+            if (
+                len(self.tokens) > 1
+                and isinstance(
+                    self.tokens[0],
+                    (ScalarToken, ValueToken, FlowSequenceEndToken, FlowMappingEndToken),
+                )
+                and isinstance(self.tokens[1], CommentToken)
+                and self.tokens[0].end_mark.line == self.tokens[1].start_mark.line
+            ):
+                self.tokens_taken += 1
+                c = self.tokens.pop(1)
+                self.fetch_more_tokens()
+                while len(self.tokens) > 1 and isinstance(self.tokens[1], CommentToken):
+                    self.tokens_taken += 1
+                    c1 = self.tokens.pop(1)
+                    c.value = c.value + (' ' * c1.start_mark.column) + c1.value
+                    self.fetch_more_tokens()
+                self.tokens[0].add_post_comment(c)
+            elif (
+                len(self.tokens) > 1
+                and isinstance(self.tokens[0], ScalarToken)
+                and isinstance(self.tokens[1], CommentToken)
+                and self.tokens[0].end_mark.line != self.tokens[1].start_mark.line
+            ):
+                self.tokens_taken += 1
+                c = self.tokens.pop(1)
+                c.value = (
+                    '\n' * (c.start_mark.line - self.tokens[0].end_mark.line)
+                    + (' ' * c.start_mark.column)
+                    + c.value
+                )
+                self.tokens[0].add_post_comment(c)
+                self.fetch_more_tokens()
+                while len(self.tokens) > 1 and isinstance(self.tokens[1], CommentToken):
+                    self.tokens_taken += 1
+                    c1 = self.tokens.pop(1)
+                    c.value = c.value + (' ' * c1.start_mark.column) + c1.value
+                    self.fetch_more_tokens()
+            self.tokens_taken += 1
+            return self.tokens.pop(0)
+        return None
+
+    def fetch_comment(self, comment):
+        # type: (Any) -> None
+        value, start_mark, end_mark = comment
+        while value and value[-1] == ' ':
+            # empty line within indented key context
+            # no need to update end-mark, that is not used
+            value = value[:-1]
+        self.tokens.append(CommentToken(value, start_mark, end_mark))
+
+    # scanner
+
+    def scan_to_next_token(self):
+        # type: () -> Any
+        # We ignore spaces, line breaks and comments.
+        # If we find a line break in the block context, we set the flag
+        # `allow_simple_key` on.
+        # The byte order mark is stripped if it's the first character in the
+        # stream. We do not yet support BOM inside the stream as the
+        # specification requires. Any such mark will be considered as a part
+        # of the document.
+        #
+        # TODO: We need to make tab handling rules more sane. A good rule is
+        #   Tabs cannot precede tokens
+        #   BLOCK-SEQUENCE-START, BLOCK-MAPPING-START, BLOCK-END,
+        #   KEY(block), VALUE(block), BLOCK-ENTRY
+        # So the checking code is
+        #   if <TAB>:
+        #       self.allow_simple_keys = False
+        # We also need to add the check for `allow_simple_keys == True` to
+        # `unwind_indent` before issuing BLOCK-END.
+        # Scanners for block, flow, and plain scalars need to be modified.
+
+        srp = self.reader.peek
+        srf = self.reader.forward
+        if self.reader.index == 0 and srp() == '\uFEFF':
+            srf()
+        found = False
+        while not found:
+            while srp() == ' ':
+                srf()
+            ch = srp()
+            if ch == '#':
+                start_mark = self.reader.get_mark()
+                comment = ch
+                srf()
+                while ch not in _THE_END:
+                    ch = srp()
+                    if ch == '\0':  # don't gobble the end-of-stream character
+                        # but add an explicit newline as "YAML processors should terminate
+                        # the stream with an explicit line break
+                        # https://yaml.org/spec/1.2/spec.html#id2780069
+                        comment += '\n'
+                        break
+                    comment += ch
+                    srf()
+                # gather any blank lines following the comment too
+                ch = self.scan_line_break()
+                while len(ch) > 0:
+                    comment += ch
+                    ch = self.scan_line_break()
+                end_mark = self.reader.get_mark()
+                if not self.flow_level:
+                    self.allow_simple_key = True
+                return comment, start_mark, end_mark
+            if bool(self.scan_line_break()):
+                start_mark = self.reader.get_mark()
+                if not self.flow_level:
+                    self.allow_simple_key = True
+                ch = srp()
+                if ch == '\n':  # empty toplevel lines
+                    start_mark = self.reader.get_mark()
+                    comment = ""
+                    while ch:
+                        ch = self.scan_line_break(empty_line=True)
+                        comment += ch
+                    if srp() == '#':
+                        # empty line followed by indented real comment
+                        comment = comment.rsplit('\n', 1)[0] + '\n'
+                    end_mark = self.reader.get_mark()
+                    return comment, start_mark, end_mark
+            else:
+                found = True
+        return None
+
+    def scan_line_break(self, empty_line=False):
+        # type: (bool) -> Text
+        # Transforms:
+        #   '\r\n'      :   '\n'
+        #   '\r'        :   '\n'
+        #   '\n'        :   '\n'
+        #   '\x85'      :   '\n'
+        #   '\u2028'    :   '\u2028'
+        #   '\u2029     :   '\u2029'
+        #   default     :   ''
+        ch = self.reader.peek()  # type: Text
+        if ch in '\r\n\x85':
+            if self.reader.prefix(2) == '\r\n':
+                self.reader.forward(2)
+            else:
+                self.reader.forward()
+            return '\n'
+        elif ch in '\u2028\u2029':
+            self.reader.forward()
+            return ch
+        elif empty_line and ch in '\t ':
+            self.reader.forward()
+            return ch
+        return ""
+
+    def scan_block_scalar(self, style, rt=True):
+        # type: (Any, Optional[bool]) -> Any
+        return Scanner.scan_block_scalar(self, style, rt=rt)
+
+
+# try:
+#     import psyco
+#     psyco.bind(Scanner)
+# except ImportError:
+#     pass