docs: function(callback) {
        fs.readdir(args['<directory>'], function(err, files) {
          // error here means it does not exist, which is good.
          if (err || args['--force']) {
            return callback(null, args['<directory>']);
          }
          if (files.length > 0) {
            return callback(new Error(format('destination '
              + 'directory %s is not empty.', path.resolve(args['<directory>']))));
          }
          return callback(null, args['<directory>']);
        });
      }