unfold

(unfold p f g seed tail-gen) ==
   (if (p seed)
       (tail-gen seed)
       (cons (f seed)
             (unfold p f g (g seed))))

public static IEnumerable<U> Unfold<T, U>(Predicate<T> p, Func<T, U> f, Func<T, T> g, T seed)
{
  if (p(seed))
    return Enumerable.Empty<U>();
  return Cons(f(seed),
              Unfold(p, f, g, g(seed)));
}

public static IEnumerable<T> Cons<T>(T car, IEnumerable<T> cdr)
{
  yield return car;
  foreach (T element in cdr) yield return element;
}