anchor.diff:  [<AutoOpen>]
   module WorldDeclarative =
   
  +    let mutable internal LensError = false
  +
       type World with
  +
  +        static member private dereferenceLensHack lens =
  +            Lens.map (fun opt ->
  +                match opt with
  +                | Some value -> value
  +                | None -> failwith "HACK: Expected lens error.")
  +                lens
   
           /// Transform a stream into existing simulants.
           static member streamSimulants
  
positive.code:                         match World.tryGetKeyedValue (scstring guid) world with
                          | Some simulant ->
                              let world = World.removeKeyedValue (scstring guid) world
                              // HACK: remove lens bindings that may depend on a non-existent model index
                              let world = world |> WorldModule.unregister simulant |> WorldModule.register simulant
                              WorldModule.destroy simulant world
                          | None -> failwithumf ())
                          world removed
  
negative.code:             Stream.insert Gen.id |>
              Stream.mapWorld (fun (guid, lenses) world ->
                  lenses |>
                  Seq.map (fun lens -> (lens.Get world, World.dereferenceLensHack lens)) |>
                  Seq.filter (fst >> Option.isSome) |>
                  Seq.take (Lens.get lensSeq world |> Seq.length) |>
                  Seq.map (fun (opt, lens) ->
  