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

theorem Th88:
  X misses Y implies (X \/ Y) \ Y = X
proof
  assume
A1: X misses Y;
  thus (X \/ Y) \ Y = (X \ Y) \/ (Y \ Y) by Th42
    .= (X \ Y) \/ {} by Lm1
    .= X by A1,Th83;
end;
