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

def iter_shape_elms(self):
        """
        Generate each child of this ``<p:spTree>`` element that corresponds
        to a shape, in the sequence they appear in the XML.
        """
        for elm in self.iterchildren():
            if elm.tag in self._shape_tags:
                yield elm