\DOC set_prompt

\TYPE {set_prompt : (string -> string)}

\SYNOPSIS
Changes the ML prompt string.

\DESCRIBE
If {s} is a string, then {set_prompt s} sets the ML prompt (which is `{#}' by
default) to the string {s}, and returns the previous prompt string. The string
may contain newlines, or be null (although {prompt false} is a cleaner way of
completely eliminating the prompt).

\FAILURE
Never fails.

\EXAMPLE
In the following session, the prompt is set to {`Ready`} on a line of its own,
then reset to its original state.
{
   #let oldprompt = set_prompt `Ready\
   #`;;
   oldprompt = `#` : string

   Ready
   do set_prompt oldprompt;;
   () : void

   # ...more ML...
}
\SEEALSO
prompt, set_lambda, set_turnstile.

\ENDDOC