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

theorem     :: ENUMSET1:4
  x in { x }
proof
  let i such that
A1: i in I;
  x.i in {x.i} by TARSKI:def 1;
  hence thesis by A1,Def1;
end;
