====== Prompt Expansion Options ======

''prompt_cr''
  * print '\r' before printing the prompt.

''prompt_subst''
  * turn on various expansions in prompts

''prompt_bang''
  * if set, '!' is treated specially in prompt expansion.

''prompt_percent''
  * if set, '%' is treated specially in prompt expansion.

''transient_rprompt''
  * remove any right prompt from display when accepting a command line. this may be useful with terminals with other cut/paste methods.

''prompt_sp''
  * attempt to preserve partial lines (no effect if 'prompt_cr' is unset).