Experimental parser: add backtracking

tools/lexer_generator/dfa.py

Index: tools/lexer_generator/dfa.py
diff --git a/tools/lexer_generator/dfa.py b/tools/lexer_generator/dfa.py
index 993575f3dbb10427058901c4ecd0bbf57f44cae1..0d4f8993300c650824420d3c470ba53d9fbc228b 100644
--- a/tools/lexer_generator/dfa.py
+++ b/tools/lexer_generator/dfa.py
@@ -158,3 +158,13 @@ class Dfa(Automaton):
 
   def terminal_set(self):
     return set(self.__terminal_set)
+
+  def build_incoming_transitions_map(self):
+    incoming_transitions = {}
+    def f(state, visitor_state):
+      for key, transition_state in state.key_state_iter():
+        if not transition_state in incoming_transitions:
+          incoming_transitions[transition_state] = []
+        incoming_transitions[transition_state].append((key, state))
+    self.visit_all_states(f)
+    return incoming_transitions