By now, you have seen and you had to write lots of programs that use the predicates append and member. What you probably did each time you needed one of them was to go back to the definition and copy it over into the file where you wanted to use it. And maybe, after having done that a couple of times, you started thinking that it was actually quite annoying that you had to copy the same predicate definitions over and over again and that it would be a lot nicer if you could define them somewhere once and for all and then just access that definition whenever you needed it. Well, that sounds like a pretty sensible thing to ask for and, of course, Prolog offers you ways of doing it.