def lookupL[K, V](k: K): List[(K, V)] @?> V = {
    @annotation.tailrec
    def lookupr(z: (List[(K, V)], (K, V), List[(K, V)])): Option[(List[(K, V)], (K, V), List[(K, V)])] =
      z match {
        case (l, x@(kk, v), r) if k == kk => Some((l, x, r))
        case (_, _, Nil)   => None
        case (l, x, r::rs) => lookupr((x::l, r, rs))
      }
    @?> {
      case Nil => None
      case h::t => lookupr((Nil, h, t)) map {
        case (l, (k, v), r) => (vv => l.reverse ::: (k, vv) :: r, v)
      }
    }
  }