gistfile1.scala

  def yipWhenL[I,O,O2](ord: (I,O) => Ordering)(f: (I,Option[O]) => O2): Wye[I,O,O2] = {
    val fbL = tee.passL[I] map (f(_,None : Option[O]))
    def go(io : Option[I], oo : Option[O]): Wye[I,O,O2] = {
      io match {
        case None    => awaitL[I].flatMap { i => go(Some(i), oo) }
        case Some(i) => oo match {
          case None     => awaitR[O].flatMap(o => go(io,Some(o))) // TODO receiveROr(fbL)...
          case Some(o)  => ord(i,o) match {
            case Ordering.LT => emit(f(i,None)) then go(None,oo)
            case Ordering.EQ => emit(f(i,oo))   then go(None,None)
            case Ordering.GT => go(io,None)
          }
        }
        
      }
    }
    go(None,None)
  }

fby is short for 'followedBy'. It's a semi-standard name in stream processing / dataflow programming. :)