Parses a complement
         Complement ::= 'complement' '(' SuperRange ')'

def _parse_complement(self, tokens):
        """ Parses a complement
         Complement ::= 'complement' '(' SuperRange ')'
        """
        tokens.pop(0)  # Pop 'complement'
        tokens.pop(0)  # Pop '('
        res = self._parse_nested_interval(tokens)
        tokens.pop(0)  # Pop ')'
        res.switch_strand()
        return res