fix-gadt9.scala

implicit val queryHFunctor: HFunctor[QueryF] = new HFunctor[QueryF] {
    def hmap[I[_], J[_]](nt: I ~> J): QueryF[I, ?] ~> QueryF[J, ?] = {
      new (QueryF[I, ?] ~> QueryF[J, ?]) {
        def apply[A](a: QueryF[I, A]): QueryF[J, A] = {
          a match {
            case QueryStringF            => QueryStringF
            case QueryBoolF              => QueryBoolF
            case query: QueryPathF[I, A] => QueryPathF(query.path, nt(query.next))
          }
        }
      }
    }
}