reserve a,b,i,j,k,l,m,n for Nat;

theorem
  for f be FinSequence, P be Permutation of (dom f) holds
    P is Permutation of (dom (Rev f))
  proof
    let f be FinSequence, P be Permutation of (dom f);
    len f = len (Rev f) by FINSEQ_5:def 3; then
    dom f = dom (Rev f) by FINSEQ_3:29;
    hence thesis;
  end;
