code: private void remove(Integer layer, Displayable displayable)
      {
          final Collection<Displayable> displayables = getLayer(layer);
          displayables.remove(displayable);
          if (displayables.isEmpty())
          {
              indexs.remove(layer);
          }
      }
docs: Remove displayable and its layer.
  
  @param layer The layer index.
  @param displayable The displayable to remove.