let hash = ref []
let rec fibomem x =
    if x < 0 then raise (Failure "fibomem") else
    if x < 2
        then 1
        else (
            try List.assoc x !hash with
            Not_found ->
                let v = fibomem(x-1) + fibomem(x-2) in
                hash := (x, v) :: !hash; v
        )