code: function(options) {
      return Promise.all(
          options.files.map(function(oneFile) {
              return fsPromise.readFile(oneFile, 'utf8').then(
                  parser.bind(null, oneFile, options)
              ).then(
                  formatter.bind(null, oneFile, options)
              ).then(
                  writer.bind(null, oneFile, options)
              );
          })
      ).spread(function() {
          return options;
      });
  }
docs: ### document the source files