theorem Th53:
  (SW-corner P)`2 = (S-min P)`2 & (SW-corner P)`2 = (S-max P)`2 &
  (S-min P)`2 = (S-max P)`2 & (S-min P)`2 = (SE-corner P)`2 & (S-max P)`2 = (
  SE-corner P)`2
proof
  (S-min P)`2 = S-bound P & (S-max P)`2 = S-bound P by EUCLID:52;
  hence thesis by EUCLID:52;
end;
