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

theorem Th79:
  X \ Y misses Y
proof
  not ex x being object st x in (X \ Y) /\ Y
  proof
    given x being object such that
A1: x in (X \ Y) /\ Y;
    x in X \ Y & x in Y by A1,XBOOLE_0:def 4;
    hence contradiction by XBOOLE_0:def 5;
  end;
  hence (X \ Y) /\ Y = {} by XBOOLE_0:def 1;
end;
