scala - parboiled2 不正なルール構成

Question

cron パーサーを書いているのですが、コンパイラが不正なルール構成を訴え、

パーサーの何が問題になっていますか?

import org.parboiled2._

sealed trait Part
case class Fixed(points: Seq[Int]) extends Part
case class Range(start: Int, end: Int) extends Part
case class Every(start: Int, interval: Int) extends Part
case object Full extends Part
case object Ignore extends Part

class CronParser(val input: ParserInput) extends Parser {

  def number = rule { capture(digits) ~> (_.toInt) }

  def digits = rule { oneOrMore(CharPredicate.Digit) }

  def fixed = rule { oneOrMore(number).separatedBy(",") ~> Fixed }

  def range = rule { digits ~ '-' ~ digits ~> Range }

  def every= rule { digits ~ '/' ~ digits ~> Every }

  def full= rule { '*' ~ push(Full) }

  def ignore = rule { '?' ~ push(Ignore) }

  def part = rule { fixed | range | every | full | ignore }

  def expr = rule { part ~ part ~ part ~ part ~ part}
}