'Return a copy of the :class:`Bbox`, shrunk by the factor *mx*
in the *x* direction and the factor *my* in the *y* direction.
The lower left corner of the box remains unchanged.  Normally
*mx* and *my* will be less than 1, but this is not enforced.'

def shrunk(self, mx, my):

    (w, h) = self.size
    return Bbox([self._points[0], (self._points[0] + [(mx * w), (my * h)])])
