worked on documentation and added an optional environment parameter

improved/parser.py

@@ -213,7 +213,20 @@ def statements():
     return Exp(statement(), sep)
 
 def parser():
+    """
+        Builds a parser for Imp.
+
+        returns -- a parser for the Imp language
+    """
     return Phrase(statements())
 
 def parse(tokens):
+    """
+        The parser entry point, takes a list of tokens and
+        transforms them into an AST.
+
+        tokens -- the token list
+
+        returns -- the AST
+    """
     return parser()(tokens, 0)