docs: 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();
      }