theorem Th29:
  (SW-corner P)`1 = (W-min P)`1 & (SW-corner P)`1 = (W-max P)`1 &
  (W-min P)`1 = (W-max P)`1 & (W-min P)`1 = (NW-corner P)`1 & (W-max P)`1 = (
  NW-corner P)`1
proof
  (W-min P)`1 = W-bound P & (W-max P)`1 = W-bound P by EUCLID:52;
  hence thesis by EUCLID:52;
end;
