(x, y, cx, cy) tuple representing net position and size.

        The values are formed as a composite of the contained child shapes.

def _child_extents(self):
        """(x, y, cx, cy) tuple representing net position and size.

        The values are formed as a composite of the contained child shapes.
        """
        child_shape_elms = list(self.iter_shape_elms())

        if not child_shape_elms:
            return Emu(0), Emu(0), Emu(0), Emu(0)

        min_x = min([xSp.x for xSp in child_shape_elms])
        min_y = min([xSp.y for xSp in child_shape_elms])
        max_x = max([(xSp.x + xSp.cx) for xSp in child_shape_elms])
        max_y = max([(xSp.y + xSp.cy) for xSp in child_shape_elms])

        x = min_x
        y = min_y
        cx = max_x - min_x
        cy = max_y - min_y

        return x, y, cx, cy