reserve m for Cardinal,
  A,B,C for Ordinal,
  x,y,z,X,Y,Z,W for set,
  f for Function;

theorem Th21:
  card Tarski-Class W <> 0 & card Tarski-Class W <> {} & card
  Tarski-Class W is limit_ordinal
proof
  thus card Tarski-Class W <> 0;
  thus card Tarski-Class W <> {};
  now
    let A;
    assume A in card Tarski-Class W;
    then A in Tarski-Class W by Th13;
    then succ A in Tarski-Class W by Th5;
    then succ A in On Tarski-Class W by ORDINAL1:def 9;
    hence succ A in card Tarski-Class W by Th9;
  end;
  hence thesis by ORDINAL1:28;
end;
