
theorem Th2:
  for n being Nat, x being object st n = {x} holds x = 0
   proof
     let n be Nat, x be object;
     assume
A1:  n = {x}; then
     card n = 1 by CARD_1:30; then
     x in {0} by A1,CARD_1:49,TARSKI:def 1;
     hence thesis by TARSKI:def 1;
   end;
