anchor.diff:  
       let handleFormDelete (form : NuEditForm) (worldChangers : WorldChangers) (_ : EventArgs) =
           ignore <| worldChangers.Add (fun world ->
  -            let selectedObject = form.propertyGrid.SelectedObject
               let world = pushPastWorld world world
  -            match selectedObject with
  +            match form.propertyGrid.SelectedObject with
               | :? EntityTypeDescriptorSource as entityTds ->
                   let world = World.destroyEntity entityTds.DescribedEntity world
                   form.propertyGrid.SelectedObject <- null
  
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:         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) = (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 ->
              match form.propertyGrid.SelectedObject with
              | null -> world
              | :? EntityTypeDescriptorSource as entityTds ->
                  let world = pushPastWorld world world
  