Generally, Prolog programs often fall into two parts. One is fixed
data - networks, family trees, and so on -
described
by structural predicates like
meets
and is_a;
the other is some component which
wanders around it, changing facts, generating output,
reading input, and so on. You have already had some practice at
expressing the fixed data, and
in Lessons 7 and 8, you will get an idea
of how to do the navigation. To prepare you for this, some
examples of what you can do by
navigating are shown in the rest of this supplement.
While reading them, you may like to bear in
mind the following quote:
A German with starving wife and family sought assistance from the Winter Aid Fund. Entering an imposing building, he found himself facing two doors, one marked ``Party Members'', the other ``Non-Party Members''. Going through the second door he was faced with two more, one marked ``Subscribers to the Fund'', and the other ``Non-Subscribers to the Fund''. Going through the latter, he was once again faced with two more doors, and so on. After an hour or two, having come to two doors, one marked ``Single Men'', the other marked ``Family Men'', he chose the latter and found himself out in the street where he had started. A friend asked him whether he had got any help. ``No'' he said, ``but heavens, what organisation!''.