code: private Collection<Displayable> getLayer(Integer layer)
      {
          final Collection<Displayable> displayables;
          if (!layers.containsKey(layer))
          {
              displayables = new HashSet<>();
              layers.put(layer, displayables);
          }
          else
          {
              displayables = layers.get(layer);
          }
          return displayables;
      }
docs: Get the layer set at the specified index. Creates an empty set if no set already defined.
  
  @param layer The layer index.
  @return The layer set reference.