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

theorem :: Theorem 2 (5)
  X in Rrank Y implies Rrank X in Rrank Y
proof
  assume X in Rrank Y;
  then the_rank_of X in the_rank_of Y by CLASSES1:66;
  hence thesis by CLASSES1:36;
end;
