@classmethod
def get_xsd_schemas(cls):
    """Get a set of distinct XSD schema URLs for this type and its
        subtypes.

        Returns
        -------
        set of str
            A set of XSD schema URLs.

        """
    xsd_schemas = set()
    fields = cls.get_fields(source='xml', include_subtypes=True)
    for f in fields.values():
        if 'xsd_type' in f:
            xsd_schemas.add(f['xsd_schema'])
    return xsd_schemas
