def _extract(self, element):
        """Extract embedded schematron schema from non-schematron host schema.
        This method will only be called by __init__ if the given schema document
        is not a schematron schema by itself.
        Must return a schematron schema document tree or None.
        """
        schematron = None
        if element.tag == _xml_schema_root:
            schematron = self._extract_xsd(element)
        elif element.nsmap[element.prefix] == RELAXNG_NS:
            # RelaxNG does not have a single unique root element
            schematron = self._extract_rng(element)
        return schematron
Extract embedded schematron schema from non-schematron host schema.
        This method will only be called by __init__ if the given schema document
        is not a schematron schema by itself.
        Must return a schematron schema document tree or None.