// python:
def factors(n):
  for p in primes():
    k = 0
    while n % (p**(k+1)) == 0: k += 1
    if k > 0: yield (p, k)

// rust??
fn factors(n: u64) -> std::iter::Map<u64, u64> {
    let cur = Cell::new(n);
    primes()
        .take_while(|&p| p <= cur.get())
        .filter(|p| cur.get() % p == 0)
        .map(|&p| (1..cur.get())
             .take_while(|&k| cur.get() % p.pow(k) == 0)
             .max()
             .map(|&k| (p, k))
             .unwrap())
}