To query or change the sequence of characters
(prompt) that indicates that the
system is waiting for user input, call prompt/2.
This predicate affects only the prompt given when a user's program
is trying to read from the terminal (for example, by calling read/1 or
get_code/1).
Note also that the prompt is reset to the default ‘|: ’ on return
to the top-level.
See mpg-ref-prompt.