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

theorem
  X c= Y implies X misses Z \ Y
proof
  assume
A1: X c= Y;
  thus X /\ (Z \ Y) = Z /\ X \ Y by Th49
    .= Z /\ (X \ Y) by Th49
    .= Z /\ {} by A1,Lm1
    .= {};
end;
