reserve X,Y,Z for set,
        x,y,z for object,
        A,B,C for Ordinal;
reserve U for Grothendieck;

theorem Th18:
  for G be Grothendieck of X holds Tarski-Class X c= G
proof
  let G be Grothendieck of X;
  G is_Tarski-Class_of X by Def4;
  hence thesis by CLASSES1:def 4;
end;
