code: def extent(self):
  		
  		if not len(self):
  			raise ValueError("empty list")
  		min, max = self[0]
  		for lo, hi in self:
  			if min > lo:
  				min = lo
  			if max < hi:
  				max = hi
  		return segment(min, max)
docs: Return the segment whose end-points denote the maximum and
  		minimum extent of the segmentlist.  Does not require the
  		segmentlist to be coalesced.