theorem Th71:
  proj1.:LSeg(NW-corner C,NE-corner C) = [.W-bound C,E-bound C.]
proof
A1: (NE-corner C)`1 = E-bound C by EUCLID:52;
  (NW-corner C)`1 = W-bound C by EUCLID:52;
  hence thesis by A1,Th21,Th52;
end;
