docs: private LineNumberReader getReader() throws FileNotFoundException {
          if (writer != null) {
              throw Context.reportRuntimeError("already writing file \""
                                               + name
                                               + "\"");
          }
          if (reader == null)
              reader = new LineNumberReader(file == null
                                          ? new InputStreamReader(System.in)
                                          : new FileReader(file));
          return reader;
      }