let fib (n) =
            if not (mem n !map) then

                let i = ref (fst (max_binding !map)) in

                let prev = ref (find !i !map) in
                let prevprev  = ref (find (!i - 1) !map) in

                while !i != n do
                    i := !i + 1;

                    map := add !i (!prev + !prevprev) !map;

                    prevprev := !prev;
                    prev := find !i !map
                done;
            find n !map