theorem
  X (@) (B2,B1) = (X` (&) (B1,B2))`
proof
  (X` (&) (B1,B2))` =((X \ ((X` (-) B1) /\ ((X`)` (-) B2)))`)` by SUBSET_1:14
    .=X \ (X (*) (B2,B1));
  hence thesis;
end;
