anchor.diff:  
       let track4
           (tracker : 'c -> 'a -> World -> 'c * bool)
  -        (plus : 'c -> 'b)
  +        (tranformer : 'c -> 'b)
           (state : 'c)
           (reactor : 'b Reactor) :
           'a Reactor =
  
positive.code:     let search p r = filter p ^^ head r
      let choose r = filter (fun o _ -> Option.isSome o) r
      let mapi r = augment (fun i _ -> i + 1) 0 r
      let max r = scan2 (fun u v _ -> if u < v then v else u) r
      let min r = scan2 (fun u v _ -> if v < u then v else u) r
      let distinct r = organize ^^ map (fun v _ -> fst v) ^^ choose r
      //let pairwise r = track (fun o _ -> match o with Some v -> (v, true) | None ->  )
  
      (* Map Combinators *)
  
  
negative.code:             let state = World.getCallbackState key world
              let (state, tracked) = tracker state value world
              let world = World.addCallbackState key state world
              if tracked then reactor.Handler (tranformer state) world
              else (Propagate, world)
          Reactor.make reactor.ReactAddress handler unsubscriber world
  
  