code: def set_prompt(self):
          
          # No right prompt
          command_line = b'PS2=;RPS1=;RPROMPT=;'
          command_line += b'PROMPT_COMMAND=;'
          command_line += b'TERM=ansi;'
          command_line += b'unset HISTFILE;'
          prompt1, prompt2 = callbacks.add(b'prompt', self.seen_prompt_cb, True)
          command_line += b'PS1="' + prompt1 + b'""' + prompt2 + b'\n"\n'
          return command_line
docs: The prompt is important because we detect the readyness of a process
          by waiting for its prompt.