// prime number filtering over an infinitie sequnce of integers.
function sieve(xs)
  | p:xs -> p:~sieve(filter(^(x) remainder(x, p) <> 0, xs))

function int_seq(start)
  start:~int_seq(start+1)

let primes = sieve(int_seq(2))

assert first(primes) == 2
assert fifth(primes) == 11
assert nth(99, primes)  == 541