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
  Y in Tarski-Class(X,A) implies bool Y in Tarski-Class(X,succ A) by Th10;
