anchor.diff:                      // compute parent position
                       let parentPosition =
                           match Option.map (resolve entity) (World.getEntityParentOpt entity world) with
  -                        | Some parent -> World.getEntityPosition parent world
  -                        | None -> Vector2.Zero
  +                        | Some parent when World.getEntityExists parent world -> World.getEntityPosition parent world
  +                        | _ -> Vector2.Zero
   
                       // update Position property
                       let world = World.setEntityPosition (parentPosition + value) entity world |> snd'
  
positive.code:             let world =
                  if changed then
                      let parentOpt = Option.map (resolve entity) (World.getEntityParentOpt entity world)
                      let visibleParent = match parentOpt with Some parent -> World.getEntityVisible parent world | None -> true
                      let visible = visibleParent && value
                      World.setEntityVisible visible entity world |> snd'
                  else world
  
negative.code:                     // compute parent elevation
                      let parentElevation =
                          match Option.map (resolve entity) (World.getEntityParentOpt entity world) with
                          | Some parent when World.getEntityExists parent world -> World.getEntityElevation parent world
                          | _ -> 0.0f
  
                      // update Elevation property
                      let world = World.setEntityElevation (parentElevation + value) entity world |> snd'
  