let rec at i = function
    | h :: t when 1 < i -> at (i - 1) t
    | h :: t when 1 = i -> Some h
    | _ -> None

at 2 [ "a" ; "b" ; "c" ; "d" ]
at -1 [ "a" ; "b" ; "c" ; "d" ]
at 2 [ "a" ]
at 2 ([ ] : int list)