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

theorem
  for n be Nat, D be set, f be D-valued FinSequence st
    n >= len f holds len(f|n) = len f
  proof
    let n be Nat, D be set, f be D-valued FinSequence;
    assume n >= len f; then
    reconsider k = n - len f as Element of NAT by  NAT_1:21;
    len (f|n) = len (f|(len f + k));
    hence thesis;
  end;
