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

theorem     :: ENUMSET1:69
  { x,x } = { x }
proof
  now
    let i be object;
    assume
A1: i in I;
    hence { x,x }.i = {x.i,x.i} by Def2
      .= {x.i} by ENUMSET1:29
      .= { x }.i by A1,Def1;
  end;
  hence thesis;
end;
