reserve i for object, I for set,
  f for Function,
  x, x1, x2, y, A, B, X, Y, Z for ManySortedSet of I;

theorem     :: ZFMISC_1:22
  {x} (\) {x,y} = EmptyMS I
proof
  now
    let i be object;
    assume
A1: i in I;
    hence ({x} (\) {x,y}).i = {x}.i \ {x,y}.i by PBOOLE:def 6
      .= {x.i} \ {x,y}.i by A1,Def1
      .= {x.i} \ {x.i,y.i} by A1,Def2
      .= {} by ZFMISC_1:16
      .= EmptyMS I.i by PBOOLE:5;
  end;
  hence {x} (\) {x,y} = EmptyMS I;
end;
