#light
type range = int * int
type input = string * int
type token =
    | Char of char
    | Identifier of string
    | Node of string * token list
    | Digit of token list

type rule = input -> (input * token list) option

let (||) r1 r2 a = match r1 a with
                        | None -> r2 a
                        | Some b -> Some b

let after res = function
    | None -> None
    | Some (a, res') -> Some (a, res' @ res)

let (&&) r1 r2 a = match r1 a with
                    | None -> None
                    | Some (a', res) -> after res (r2 a')

let char c (str:string, pos) =
    try
        if str.[pos] = c then Some ((str, pos + 1), [Char c]) else None
    with _ -> None

let rec loop rule a = (rule && loop rule || rule) a

let nnode c rule a = match rule a with
                                | None -> None
                                | Some (a', res) -> Some (a', [c (res)])

let bin =
    let digit = char '0' || char '1' in
    nnode (function x -> Digit x) (loop digit)