theorem Th73:
  proj1.:LSeg(SE-corner C,SW-corner C) = [.W-bound C,E-bound C.]
proof
A1: (SW-corner C)`1 = W-bound C by EUCLID:52;
  (SE-corner C)`1 = E-bound C by EUCLID:52;
  hence thesis by A1,Th21,Th52;
end;
