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

theorem     :: ENUMSET1:3
  x in { A } implies x = A
proof
  assume
A1: x in { A };
  now
    let i be object;
    assume
A2: i in I;
    then x.i in {A}.i by A1;
    then x.i in {A.i} by A2,Def1;
    hence x.i = A.i by TARSKI:def 1;
  end;
  hence thesis;
end;
