theorem Th6:
  p in W-most D implies p`1 = W-bound D
proof
  assume p in W-most D;
  then
A1: p in LSeg(SW-corner D, NW-corner D) by XBOOLE_0:def 4;
  (SW-corner D)`1 = W-bound D by EUCLID:52
    .= (NW-corner D)`1 by EUCLID:52;
  hence p`1 = (SW-corner D)`1 by A1,GOBOARD7:5
    .= W-bound D by EUCLID:52;
end;
