code: public ProcessExecutor stopper(ProcessStopper stopper) {
      if (stopper == null) {
        stopper = NopProcessStopper.INSTANCE;
      }
      this.stopper = stopper;
      return this;
    }
docs: Sets the helper for stopping the process in case of timeout or cancellation.
  <p>
  By default {@link DestroyProcessStopper} is used which just invokes {@link Process#destroy()}.
  
  @param stopper helper for stopping the process (<code>null</code> means {@link NopProcessStopper} - process is not stopped).
  @return This process executor.