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

theorem
  for f be FinSequence holds
    idseq (len f) is Permutation of (dom f)
proof
  let f be FinSequence;
  Seg (len f) = dom f by FINSEQ_1:def 3;
  hence thesis by FINSEQ_2:55;
end;
