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
  Tarski-Class(X,succ A) = { u : ex v st v in Tarski-Class(X,A) & u c= v } \/
  { bool v : v in Tarski-Class(X,A) } \/
  bool Tarski-Class(X,A) /\ Tarski-Class X by Lm1;
