special markers
     'RL_PROMPT_START_IGNORE' and 'RL_PROMPT_END_IGNORE' (declared in
     'readline.h' as '\001' and '\002', respectively).  This may be used
     to embed terminal-specific escape sequences in prompts.

 -- Function: int rl_set_prompt (const char *prompt)
     Make Readline use PROMPT for subsequent redisplay.  This calls
     'rl_expand_prompt()' to expand the prompt and sets 'rl_prompt' to
     the result.