New-line in prompt

["Owen M. Astley" <oma1000@xxxxxxxxx>]

I am trying to get a new line (\n) in a prompt (actually I only want the
new line if the current one is too long).  Has anybody got any idea about
how to do this?  %{\n%} works but, as the man page says, it shouldn't
change the cursor position (for obvious reasons if you use it).

Thanks, Owen