N^th Prime Number

Implement the function

val prime : int -> int

so that prime n is the \(n\)^th prime number. The behavior of this function is undefined if n is negative.

let _ = assert (prime 0 = 2)
let _ = assert (prime 1 = 3)
let _ = assert (prime 2 = 5)
let _ = assert (prime 500 = 3581)