%  =================================================================
%  ==                                                             ==
%  ==          An Introduction to ARTIFICIAL INTELLIGENCE         ==
%  ==                  Janet Finlay and Alan Dix                  ==
%  ==                       UCL Press, 1996                       ==
%  ==                                                             ==
%  =================================================================
%  ==                                                             ==
%  ==           chapter 1, page 16:  representing logic           ==
%  ==                                                             ==
%  ==            Prolog example, Alan Dix, August 1996            ==
%  ==                                                             ==
%  =================================================================


%  The family relationships for this example are drawn from the
%  Thunderbirds television series.  See the web pages for more
%  information about Thunderbirds.


father(jeff,virgil).     %  Note the use of lower case names
father(jeff,alan).       %  If we wrote  'father(Jeff,Alan)' in Prolog
father(jeff,scott).      %  the upper case names 'Jeff' and 'Alan'
father(jeff,john).       %  would be treated as variables, just as if
father(jeff,gordon).     %  we had written 'father(X,Y)'.  That is we
mother(grandma,jeff).    %  we would have said that everybody was
father(kyrano,tin_tin).  %  everybody else's father!

sibling(kyrano,hood).  %  Kyrano and the Hood are declared as siblings as
sibling(hood,kyrano).  %  we do not know who their father or mother is.
                       %  This shows how Prolog allows you to mix declared
                       %  facts such as these with rules (below) for the
                       %  same predicate.

parent(X,Y) :- father(X,Y).
parent(X,Y) :- mother(X,Y).

sibling(Y,Z) :- parent(X,Y), parent(X,Z).

    %  There are two facts given about the Hood and Kyrano.
    %  Why don't I have rule saying 'sibling(Y,Z) :- sibling(Z,Y)',
    %  as this would have allowed us to deduce 'sibling(hood,kyrano)'
    %  from the fact 'sibling(kyrano,hood)'?
    %  If you aren't sure, try adding this rule and see what happens!


%  RUNNING THIS CODE
%
%  When testing any program, always remember to try test cases of both
%  things you expect to work:  e.g.  'sibling(alan,john)'
%  and those you don't:        e.g.  'sibling(hood,grandma)'
%
%  Also with Prolog you can try asking it questions with variables
%  in such as: 'sibling(scott,X)'
%
%  You may get a surprise, among the answers you will get
%  'sibling(scott,scott)'.  Actually this makes perfect sense.
%  'jeff' is the parent of 'scott' and so by the rule 'scott'
%  is his own sibling.
%
%  To see all the sibling pairs try asking:  'sibling(X,Y)'
%  Again you will see all the reflexive pairs 'sibling(virgil,virgil' etc.
%  and also you will see answers both ways round for each pair.