code: function writer(err, collections) {
      if (err) return done(err);
  
      // @TODO make sure that self.write only accepts 2 arguments and reads the
      // type from the supplied collection object.
      self.logger.debug('Writing files');
      async.map(collections, self.write.bind(self), done);
    }
docs: Write the processed collections to disk.
  
  @param {Error} err
  @param {Array} collections