function plugin() {
        /**
         * @param {Root} tree
         *   Tree.
         * @returns {undefined}
         *   Nothing.
         */
        return function (tree) {
          tree.children.push({
            type: 'element',
            tagName: 'a',
            properties: {
              className: 'b',
              style: '-webkit-box-shadow: 0 0 1px 0 red'
            },
            children: []
          })
        }
      }
@param {Root} tree
  Tree.
@returns {undefined}
  Nothing.