docs: function getAddMtime(file, done) {
      fs.stat(path.join(source, file), addMtime);
  
  
      /**
       * Adds `stats.mtime` of `file` to its metadata.
       *
       * @param {Error} err
       * @param {fs.Stats} stats
       * @api private
       */
  
      function addMtime(err, stats) {
        if (err) {
          // Skip elements of `files` that don't point to existing files.
          // This can happen if some other Metalsmith plugin does something
          // strange with `files`.
          if (err.code === 'ENOENT') {
            debug('file %s not found', file);
            return done();
          }
          return done(err);
        }
  
        debug('mtime of file %s is %s', file, stats.mtime);
        files[file].mtime = stats.mtime;
        done();
      }
    }