code: public static void exports(Xml root, Collidable collidable)
      {
          Check.notNull(root);
          Check.notNull(collidable);
  
          final Xml node = root.createChild(NODE_GROUP);
          node.setText(collidable.getGroup().toString());
      }
docs: Create an XML node from a collidable.
  
  @param root The node root (must not be <code>null</code>).
  @param collidable The collidable reference (must not be <code>null</code>).
  @throws LionEngineException If invalid argument.