theorem Th3:
  p in N-most D implies p`2 = N-bound D
proof
  assume p in N-most D;
  then
A1: p in LSeg(NW-corner D, NE-corner D) by XBOOLE_0:def 4;
  (NE-corner D)`2 = N-bound D by EUCLID:52
    .= (NW-corner D)`2 by EUCLID:52;
  hence p`2 = (NW-corner D)`2 by A1,GOBOARD7:6
    .= N-bound D by EUCLID:52;
end;
