code: function IndexedNode(edit, mask, children) {
      return {
          type: INDEX,
          edit: edit,
          mask: mask,
          children: children,
          _modify: IndexedNode__modify
      };
  }
docs: Internal node with a sparse set of children.
  
  Uses a bitmap and array to pack children.
  
  @member edit Edit of the node.
  @member mask Bitmap that encode the positions of children in the array.
  @member children Array of child nodes.