reserve W,X,Y,Z for set,
  f,g for Function,
  a,x,y,z for set;
reserve u,v for Element of Tarski-Class(X),
  A,B,C for Ordinal,
  L for Sequence;

theorem Th19:
  ex A st Tarski-Class(X,A) = Tarski-Class X
proof
  consider A such that
A1: Tarski-Class(X,A) = Tarski-Class(X,succ A) by Th17;
  take A;
  thus thesis by A1,Th18;
end;
