code: func (s *Session) StderrPipe() (io.Reader, error) {
  	if s.Stderr != nil {
  		return nil, errors.New("ssh: Stderr already set")
  	}
  	if s.started {
  		return nil, errors.New("ssh: StderrPipe after process started")
  	}
  	s.stderrpipe = true
  	return s.ch.Stderr(), nil
  }
docs: // StderrPipe returns a pipe that will be connected to the
  // remote command's standard error when the command starts.
  // There is a fixed amount of buffering that is shared between
  // stdout and stderr streams. If the StderrPipe reader is
  // not serviced fast enough it may eventually cause the
  // remote command to block.