code: def extent_all(self):
  		
  		segs = tuple(seglist.extent() for seglist in self.values() if seglist)
  		if not segs:
  			raise ValueError("empty list")
  		return segment(min(seg[0] for seg in segs), max(seg[1] for seg in segs))
docs: Return the result of running .extent() on the union of all
  		lists in the dictionary.