function plugin() {
        /**
         * @param {Root} tree
         *   Tree.
         * @returns {undefined}
         *   Nothing.
         */

        return function (tree) {
          tree.children.push({
            type: 'element',
            tagName: 'custom-element',
            properties: {},
            children: []
          })
        }
      }
@param {MdastRoot} tree
  Tree.
@returns {undefined}
  Nothing.