intsfrom a == a : intsfrom (a + 1)

first n [] == []
first 0 x == []
first n (a : b) == a : first (n - 1) b

notdiv n == { x -> x % n != 0 }

filter f [] == []
filter f (a : b) == a : x <| f a |> x where { x == filter f b }

era (a : b) == a : era (filter (notdiv a) b)

primes == era (intsfrom 2)