Return a multivalued parser for the given tag (e.g. 020 or 700.a).

def tag_to_expression(self, tag, prefix=None, index=None):
        """ Return a multivalued parser for the given tag (e.g. 020 or 700.a).
        """
        if prefix is None:
            prefix = self.default_prefix
        if index is None:
            index = self.default_index
        tag = tag.replace('.', '').strip()
        if 3 > len(tag) > 4:
            raise ValueError('tag must be of the form 008, 020a or 020.a')
        if len(tag) == 4:
            tag, code = tag[:3], tag[3:]
            return jpath.parse('{prefix}.content["{tag}"]'
                               '[{index}].["{code}"]'.format(prefix=prefix,
                                                             tag=tag, index=index, code=code))
        else:
            return jpath.parse('{prefix}.content["{tag}"]'.format(prefix=prefix,
                                                                  tag=tag))