reserve X,Y,x for set;
reserve A for non empty preBoolean set;

theorem Th13:
  for A being set holds Fin A c= bool A
proof
  let A be set;
  let x be object;
  reconsider xx=x as set by TARSKI:1;
  assume x in Fin A;
  then xx c= A by Def5;
  hence thesis;
end;
