//looks for an html comment pattern, and, if found,
//adds the text before it to the doc under the key `summary`.
const reg = /.*<!--summary-start-->(?<summary>.+)?<!--summary-end-->/s
//use a custom element instead.
// const reg = /<doc-summary>(?<summary>.+)?<\/doc-summary>/s
export default (doc) => {
  const result = reg.exec(doc._body);
  if (result) {
    doc.summary = result.groups.summary;
  }
  return doc;
}