Get the anchor's index.
        This must return an ``int``.

        Subclasses may override this method.

def _get_index(self):
        """
        Get the anchor's index.
        This must return an ``int``.

        Subclasses may override this method.
        """
        glyph = self.glyph
        if glyph is None:
            return None
        return glyph.anchors.index(self)