theorem Th72:
  proj2.:LSeg(NE-corner C,SE-corner C) = [.S-bound C,N-bound C.]
proof
A1: (SE-corner C)`2 = S-bound C by EUCLID:52;
  (NE-corner C)`2 = N-bound C by EUCLID:52;
  hence thesis by A1,Th22,Th53;
end;
