Provides all XML documents for that content type
        @param document: a Document or subclass object
        @param content_type: a MIME content type
        @return: list of etree._ElementTree of that content type

def getTreesFor(self, document, content_type):
        """Provides all XML documents for that content type
        @param document: a Document or subclass object
        @param content_type: a MIME content type
        @return: list of etree._ElementTree of that content type
        """

        # Relative path without potential leading path separator
        # otherwise os.path.join doesn't work
        for rel_path in self.overrides[content_type]:
            if rel_path[0] in ('/', '\\'):
                rel_path = rel_path[1:]
            file_path = os.path.join(document._cache_dir, rel_path)
            yield etree.parse(utils.xmlFile(file_path, 'rb'))
        return