anchor.diff:  
   namespace Nu
   open System
  -open System.Collections
  -open System.Collections.Generic
   open Prime
   open Nu
   
  
positive.code:             let monitorMapper =
                  fun a _ world ->
                      let b =
                          if refEq a.Value monitorResult || genEq a.Value monitorResult then
                              match sieveResultOpt with
                              | Some b -> b
                              | None -> sieve (lens.Get world)
                          else sieve (lens.Get world)
                      monitorResult <- a.Value
                      sieveResultOpt <- Some b
  
negative.code:                     | None -> world
                  | None -> world)
                  world removed
  
          static member internal addSynchronizedSimulants mapper monitorMapper simulantMapId added origin owner parent world =
              Seq.fold (fun world keyAndLens ->
                  let (key, lens) = PartialComparable.unmake keyAndLens
  