@classmethod
def extend_fields(cls, extra_fields):
    """Extend the fields of this type with given extra fields and return
        the new fieldset.

        Parameters
        ----------
        extra_fields : list of pydov.types.fields.AbstractField
            Extra fields to be appended to the existing fields of this type.

        Returns
        -------
        list of pydov.types.fields.AbstractField
            List of the existing fields of this type, extended with the
            extra fields supplied in extra_fields.

        """
    fields = list(cls.fields)
    fields.extend(extra_fields)
    return fields
