Generate each child of the ``<p:spTree>`` element that corresponds to
        a shape, in the sequence they appear in the XML.

def _iter_member_elms(self):
        """
        Generate each child of the ``<p:spTree>`` element that corresponds to
        a shape, in the sequence they appear in the XML.
        """
        for shape_elm in self._spTree.iter_shape_elms():
            if self._is_member_elm(shape_elm):
                yield shape_elm