theorem Th50:
  X "/\" Y misses X \ Y
proof
  (X "/\" Y) "/\" (X \ Y) = (X "/\" Y "/\" Y`) "/\" X by LATTICES:def 7
    .= (X "/\" (Y "/\" Y`)) "/\" X by LATTICES:def 7
    .= Bottom L "/\" X by LATTICES:20
    .= Bottom L;
  hence thesis;
end;
