reserve x,A,B,X,X9,Y,Y9,Z,V for set;

theorem Th89:
  X /\ Y misses X \ Y
proof
  now
    let x be object;
    not (x in X & x in Y & not x in Y);
    hence not (x in X /\ Y & x in X \ Y) by XBOOLE_0:def 4,def 5;
  end;
  hence thesis by XBOOLE_0:3;
end;
