front_lastLib.front_last : 'a list -> 'a list * 'a
Takes a non-empty list L and returns a pair
(front,last) such that front @ [last] = L.
Fails if the list is empty.
> front_last [1];
val it = ([], 1): int list * int
> front_last [1,2,3];
val it = ([1, 2], 3): int list * int