Return newly-added `p:sp` element as specified.

        `p:sp` element is of *autoshape_type* at position (*x*, *y*) and of
        size (*cx*, *cy*).

def _add_sp(self, autoshape_type, x, y, cx, cy):
        """Return newly-added `p:sp` element as specified.

        `p:sp` element is of *autoshape_type* at position (*x*, *y*) and of
        size (*cx*, *cy*).
        """
        id_ = self._next_shape_id
        name = '%s %d' % (autoshape_type.basename, id_-1)
        sp = self._grpSp.add_autoshape(
            id_, name, autoshape_type.prst, x, y, cx, cy
        )
        return sp