added ast definition and parser

improved/parser_combinator.py

@@ -155,7 +155,7 @@ class Concat(Parser):
                 return Result(combined_value, right_result.pos)
         return None
 
- class Alternate(Parser):
+class Alternate(Parser):
     """The alternate combinator. Parses using either of two parsers."""
     def __init__(self, left, right):
         """