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

theorem Th4:
  W is Tarski & X in W implies Tarski-Class X c= W
proof
  assume that
A1: W is Tarski and
A2: X in W;
  reconsider D = W as non empty set by A2;
  D is_Tarski-Class_of X by A1,A2;
  hence thesis by CLASSES1:def 4;
end;
