* an input consuming parser at its front (e.g., in `p = paren(p) + p`, the `p` at the right would
 * cause an infinite recursion).
 */
internal
fun <T> reference(): ParserRef<T> = ParserRef()


internal
fun <T> zeroOrMore(parser: Parser<T>): Parser<List<T>> = {
    val lazyResult: MutableList<T>? = orMore(parser)
    ParserResult.Success(lazyResult ?: emptyList())
}