What general tips do you have for golfing in Prolog? I am looking for ideas that can be applied to code golf problems in general that are at least somewhat specific to Prolog (e.g. one letter variables is not specific to Prolog to reduce the size of programs).
Please indicate in your tips if it is specific to an implementation of Prolog (e.g. SWI-Prolog specific built-ins)
Please post only one tip per answer, or a list of tips that are all closely related to the same main idea.