Repeats

Implement the function

val repeats : ('a * int) list -> 'a option list

so that repeats l is the result of replacing each tuple (x, n) with n copies of Some x in the case that n is nonnegative, and -n copies of None otherwise. Your implementation must be tail recursive.

let _ = assert (repeats [(true, 3)] = [Some true; Some true; Some true])
let _ = assert (repeats [(1, 2); (2, 0); (3, -3)] = [Some 1; Some 1; None; None; None])