The Common Prolog system comes with a built-in read-query-print loop similar to a Prolog interpreter loop. To run it, make sure the common-prolog package is accessible and type: (rqp). You will be presented with the prompt: ==>. At this point you may type in goal expressions, for example:
|==> (append ?x ?y (1 2))
|
|?X = NIL
|?Y = (1 2)
Now Common Prolog is waiting for you to indicate whether or not you wish more solutions. If you press Return, you will get the message OK and return to the top level:
|?X = NIL
|?Y = (1 2)<RETURN>
|
|OK.
|
|==>
KnowledgeWorks and Prolog User Guide (Unix version) - 26 Feb 2015