private def tr[T](pf:PartialFunction[Elem,Elem]): T=>T ={
      case e:Elem if pf.isDefinedAt(e) => pf(e).asInstanceOf[T]
      case e:Elem =>{
        val xs=e.child.map(tr(pf))
        val(a,b)=e.child.zip(xs).partition(x=>x._1==x._2)
        (if(b.isEmpty) e else e.copy(child=xs)).asInstanceOf[T]
      }
      case x => x
    }