hasdiv n == hasdiv1 (n - 1) n
hasdiv1 0 n == false
hasdiv1 1 n == false
hasdiv1 a n == true <| n % a = 0 |> hasdiv1 (a - 1) n

prime 1 == false
prime n == not (hasdiv n)

primes == pfr 2

pfr n == n : rest <| prime n |> rest   where { rest == pfr (n + 1) }