code: private static Integer getLayer(Featurable featurable)
      {
          if (featurable.hasFeature(Layerable.class))
          {
              final Layerable layerable = featurable.getFeature(Layerable.class);
              return layerable.getLayerDisplay();
          }
          return LAYER_DEFAULT;
      }
docs: Get the featurable layer.
  
  @param featurable The featurable reference.
  @return The featurable layer if is {@link Layerable}, {@link #LAYER_DEFAULT} else.