anchor.diff:              (origin : ContentOrigin)
               (parent : Simulant)
               world =
  -            let mutable previous = Set.empty // TODO: P1: consider if a USet or HashSet would be more efficient now that HashSet has property cloning in its ctor
               let guid = Gen.id
               let sieve' = fun a ->
                   match lens.PayloadOpt with
  
positive.code:                 let added = Set.difference current previous
                  let removed = Set.difference previous current
                  let changed = Set.notEmpty added || Set.notEmpty removed
                  previous <- current
                  let world =
                      if changed then
                          let world =
  
negative.code:             let lensSeq = lens |> Lens.map sieve |> Lens.mapWorld unfold
              let lenses = Lens.explodeIndexedOpt indexOpt lensSeq
              let subscription = fun _ world ->
                  let previous =
                      match World.tryGetKeyedValue<PartialComparable<Guid, int * Lens<obj, World>> Set> guid world with
                      | Some previous -> previous
                      | None -> Set.empty
                  let mutable current = Set.empty
                  let mutable count = Lens.get lensSeq world |> Seq.length
                  let mutable enr = lenses.GetEnumerator ()
  