Prime sequence

let isPrime = function
    | 0L | 1L -> false
    | n ->
        let h = n / 2L
        let rec f = function
            | x when x > h -> true
            | x when n % x = 0L -> false
            | x -> f (x + 1L)
        f 2L

let primes =
    let rec primes n = seq {
        if isPrime n then
            yield n
        yield! primes (n + 1L)
    }
    primes 2L