Set prompt associated with read/1
and its derivatives. Old is first unified with the current
prompt. On success the prompt will be set to New if this is
an atom. Otherwise an error message is displayed. A prompt is printed if
one of the read predicates is called and the cursor is at the left
margin. It is also printed whenever a newline is given and the term has
not been terminated. Prompts are only printed when the current input
stream is user.