Tokenizes and parses an arithmetic expression into a parse tree.

        @return: Returns a token string.
        @rtype: lems.parser.expr.ExprNode

def parse(self):
        """
        Tokenizes and parses an arithmetic expression into a parse tree.

        @return: Returns a token string.
        @rtype: lems.parser.expr.ExprNode
        """
        #print("Parsing: %s"%self.parse_string)
        self.tokenize()
        if self.debug: print("Tokens found: %s"%self.token_list)
        
        try:
            parse_tree = self.parse2()
        except Exception as e:
                raise e
        return parse_tree