--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/build_ucsc_custom_track_code.py	Mon Jul 28 11:55:41 2014 -0400
@@ -0,0 +1,21 @@
+# runs after the job (and after the default post-filter)
+
+# Older py compatibility
+try:
+    set()
+except:
+    from sets import Set as set
+
+def validate_input( trans, error_map, param_values, page_param_map ):
+    dbkeys = set()
+    tracks = param_values['tracks']
+    for track in tracks:
+        if track['input']:
+            dbkeys.add( track['input'].dbkey )
+    if len( dbkeys ) > 1:
+        # FIXME: Should be able to assume error map structure is created
+        if 'tracks' not in error_map:
+            error_map['tracks'] = [ dict() for t in tracks ]
+            for i in range( len( tracks ) ):
+                error_map['tracks'][i]['input'] = \
+                    "All datasets must belong to same genomic build"
\ No newline at end of file