code: func (s *Session) Start(cmd string) error {
  	if s.started {
  		return errors.New("ssh: session already started")
  	}
  	req := execMsg{
  		Command: cmd,
  	}
  
  	ok, err := s.ch.SendRequest("exec", true, Marshal(&req))
  	if err == nil && !ok {
  		err = fmt.Errorf("ssh: command %v failed", cmd)
  	}
  	if err != nil {
  		return err
  	}
  	return s.start()
  }
docs: // Start runs cmd on the remote host. Typically, the remote
  // server passes cmd to the shell for interpretation.
  // A Session only accepts one call to Run, Start or Shell.