Return relationship of matching *reltype*, *target*, and
        *is_external* from collection, or None if not found.

def _get_matching(self, reltype, target, is_external=False):
        """
        Return relationship of matching *reltype*, *target*, and
        *is_external* from collection, or None if not found.
        """
        def matches(rel, reltype, target, is_external):
            if rel.reltype != reltype:
                return False
            if rel.is_external != is_external:
                return False
            rel_target = rel.target_ref if rel.is_external else rel.target_part
            if rel_target != target:
                return False
            return True

        for rel in self.values():
            if matches(rel, reltype, target, is_external):
                return rel
        return None