'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.'

def _extract(self, element):

    schematron = None
    if (element.tag == _xml_schema_root):
        schematron = self._extract_xsd(element)
    elif (element.nsmap[element.prefix] == RELAXNG_NS):
        schematron = self._extract_rng(element)
    return schematron
