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

theorem Th34:
  X c= Y implies Z \ Y c= Z \ X
proof
  assume
A1: X c= Y;
  let x be object;
  assume
A2: x in Z \ Y;
  then not x in Y by XBOOLE_0:def 5;
  then
A3: not x in X by A1;
  x in Z by A2,XBOOLE_0:def 5;
  hence thesis by A3,XBOOLE_0:def 5;
end;
