Performs parsing process against token stream to generate the
            internal abstract syntax tree (AST) for general purpose use.

            * Raises a ``ParseError`` exception upon failure.

def _parse(self):
        """ Performs parsing process against token stream to generate the
            internal abstract syntax tree (AST) for general purpose use.

            * Raises a ``ParseError`` exception upon failure.
            """
        try:
            # parse token stream into abstract syntax tree (AST)
            self._ast = self._rule_container()

        except ParseError:
            raise

        except Exception as exc:
            raise ParseError(u'Unexpected error: {0}'.format(unicode(exc)))