theorem Th37:
  (NW-corner P)`2 = (N-min P)`2 & (NW-corner P)`2 = (N-max P)`2 &
  (N-min P)`2 = (N-max P)`2 & (N-min P)`2 = (NE-corner P)`2 & (N-max P)`2 = (
  NE-corner P)`2
proof
  (N-min P)`2 = N-bound P & (N-max P)`2 = N-bound P by EUCLID:52;
  hence thesis by EUCLID:52;
end;
