theorem Th5:
  p in S-most D implies p`2 = S-bound D
proof
  assume p in S-most D;
  then
A1: p in LSeg(SW-corner D, SE-corner D) by XBOOLE_0:def 4;
  (SE-corner D)`2 = S-bound D by EUCLID:52
    .= (SW-corner D)`2 by EUCLID:52;
  hence p`2 = (SW-corner D)`2 by A1,GOBOARD7:6
    .= S-bound D by EUCLID:52;
end;
