rosetree.scala

  def tree[F[_[_]]](implicit T: BirecursiveT[F]): Algebra[SExp, F[RTree[String, ?]]] = {
    case Nil => RTree.Node(List.empty[F[RTree[String, ?]]]).embed
    case True => RTree.Leaf("t").narrow[F[RTree[String, ?]]].embed
    case Int(v) => RTree.Leaf(v.toString).narrow[F[RTree[String, ?]]].embed
    case Keyword(k) => RTree.Leaf(s":$k").narrow[F[RTree[String, ?]]].embed
    case Symbol(s) => RTree.Leaf(s"'$s").narrow[F[RTree[String, ?]]].embed
    case Char(c) => RTree.Leaf(s"?$c").narrow[F[RTree[String, ?]]].embed  
    case Pair(l, r) => r.project match {
       case RTree.Node(t) => RTree.Node(l :: t).embed
       case RTree.Leaf(r) => RTree.Leaf(s"($l . $r)").narrow[F[RTree[String, ?]]].embed
    }
  }

  val show: Algebra[RTree[String, ?], String] = {
    case Node(Nil) => "nil"
    case Node(xs) => s"""(${xs.mkString(" ")})"""  
    case Leaf(s) => s
  }