Experimental parser: add backtracking

tools/lexer_generator/transition_key.py

 # Copyright 2013 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 385 matching lines @@
     return chain(*map(lambda x : x.__flatten(), keys))
 
   @staticmethod
   def __disjoint_components_from_keys(encoding, keys, merge_ranges = False):
     return TransitionKey.__disjoint_components(
       encoding, TransitionKey.__flatten_keys(keys), merge_ranges)
 
   @staticmethod
   def __disjoint_components(encoding, components, merge_ranges):
     range_map = {}
-    other_keys = set([])
+    other_keys = set()
     for x in components:
       if x.name() != 'NUMERIC_RANGE_KEY':
         other_keys.add(x)
         continue
       (start, end) = x.args()
       if not start in range_map:
         range_map[start] = []
       range_map[start].append(end)
     ranges = TransitionKey.__disjoint_keys(encoding, range_map)
     if merge_ranges:
@@ skipping 57 matching lines @@
         yield key(last[1] + 1, r[0] - 1)
       last = r
     upper_bound = encoding.primary_range()[1]
     if last == None:
       r = encoding.primary_range()
       yield key(r[0], r[1])
     elif last[1] < upper_bound:
       yield key(last[1] + 1, upper_bound)
     for c in sorted(classes, TransitionKey.__component_compare):
       yield c