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;
reserve n for Element of omega;

theorem
  the_rank_of Rank A = A
proof
 for B st Rank A c= Rank B holds A c= B by Th37;
  hence thesis by Def9;
end;
