
theorem Th2:
  for X being set, x being object st x in X & card X = 1 holds {x} = X
proof
  let X be set, x be object;
  assume that
    A1: x in X and
    A2: card X = 1;
  consider y being object such that
B1:  X = {y} by CARD_2:42,A2;
  x in {y} by B1,A1; then
  x = y by TARSKI:def 1;
  hence thesis by B1;
end;
