theorem Th33:
  X \ X "/\" Y = X \ Y
proof
  X \ X "/\" Y = X "/\" (X` "\/" Y`) by LATTICES:23
    .= (X "/\" X`) "\/" (X "/\" Y`) by LATTICES:def 11
    .= Bottom L "\/" (X "/\" Y`) by LATTICES:20
    .= X "/\" Y`;
  hence thesis;
end;
