anchor.diff:          ignore <| worldChangers.Add (fun world ->
               // TODO: allow disabling of physics as well
               let interactivity = if form.interactivityButton.Checked then GuiAndPhysicsAndGamePlay else GuiAndPhysics
  -            let pastWorld = world
  -            let world = World.setInteractivity interactivity world
  -            if Interactivity.isGamePlaying interactivity then pushPastWorld pastWorld world
  +            let (pastWorld, world) = (world, World.setInteractivity interactivity world)
  +            if Interactivity.isGamePlaying interactivity
  +            then pushPastWorld pastWorld world
               else world)
   
       (*let handleFormCut (form : NuEditForm) (worldChangers : WorldChangers) (_ : EventArgs) =
           ignore <| worldChangers.Add (fun world ->
  -            let optEntityTds = form.propertyGrid.SelectedObject
  -            match optEntityTds with
  +            match form.propertyGrid.SelectedObject with
               | null -> world
               | :? EntityTypeDescriptorSource as entityTds ->
                   let world = pushPastWorld world world
  
positive.code:         
      let handleFormCopy (form : NuEditForm) (worldChangers : WorldChangers) (_ : EventArgs) =
          ignore <| worldChangers.Add (fun world ->
              let optEntityTds = form.propertyGrid.SelectedObject
              match optEntityTds with
              | null -> world
              | :? EntityTypeDescriptorSource as entityTds ->
                  let entity = World.getEntity entityTds.Address world
  
negative.code: 
      let handleFormDelete (form : NuEditForm) (worldChangers : WorldChangers) (_ : EventArgs) =
          ignore <| worldChangers.Add (fun world ->
              let world = pushPastWorld world world
              match form.propertyGrid.SelectedObject with
              | :? EntityTypeDescriptorSource as entityTds ->
                  let world = World.destroyEntity entityTds.DescribedEntity world
                  form.propertyGrid.SelectedObject <- null
  