theorem Th4:
  p in E-most D implies p`1 = E-bound D
proof
  assume p in E-most D;
  then
A1: p in LSeg(SE-corner D, NE-corner D) by XBOOLE_0:def 4;
  (SE-corner D)`1 = E-bound D by EUCLID:52
    .= (NE-corner D)`1 by EUCLID:52;
  hence p`1 = (SE-corner D)`1 by A1,GOBOARD7:5
    .= E-bound D by EUCLID:52;
end;
