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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sky_bbox_ll(self):
 """ The sky coordinates of the lower-left vertex of the minimal bounding box of the source segment, returned as a `~astropy.coordinates.SkyCoord` object. The bounding box encloses all of the source segment pixels in their entirety, thus the vertices are at the pixel *corners*. """


        if self._wcs is not None:
            return pixel_to_skycoord(self.xmin.value - 0.5,
                                     self.ymin.value - 0.5,
                                     self._wcs, origin=0)
        else:
            return None