var processed = [];
  expandDirectories(indexes, filterer).forEach(function (index) {
    // support { source, file } input
    if (typeof index === 'object' && index.source) {
      processed.push(index);
      return;
    }

    md.write(path.resolve(index));
  });
  md.end();
  md.once('error', function (error) {
    return callback(error);
  });
  md.pipe(concat(function (inputs) {
    callback(null, inputs.map(function (input) {
      // un-transform babelify transformed source
      input.source = fs.readFileSync(input.file, 'utf8');
      return input;
    }).concat(processed));
  }));