code: def mask(self, pattern):
          
          cairo.cairo_mask(self._pointer, pattern._pointer)
          self._check_status()
docs: A drawing operator that paints the current source
          using the alpha channel of :obj:`pattern` as a mask.
          (Opaque areas of :obj:`pattern` are painted with the source,
          transparent areas are not painted.)
  
          :param pattern: A :class:`Pattern` object.