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

theorem :: Theorem 2 (7)
  Rrank X in Rrank Y or Rrank Y c= Rrank X
proof
  assume not Rrank X in Rrank Y;
  then not the_rank_of X in the_rank_of Y by CLASSES1:36;
  hence thesis by CLASSES1:37,ORDINAL1:16;
end;
