async function combine(insert, source, filename) {
  // `/s` = . matches new line

  // console.log(insert);
  // console.log('-'.repeat(50));

  const result = source.replace(
    /<!--oc-->.+<!--oc-->/s,
    `<!--oc-->${insert}<!--oc-->`
  );
  await writeFile(filename, result);
}
@param {string} insert
@param {string} source
@param {string} filename