% family.pl

% British Royal Family

mother(victoria, edward).
father(albert, edward).
mother(victoria, alice).
father(albert, alice).

% Greek Gods

father(zeus, ares).
mother(hera, ares).
father(ares, harmonia).
mother(aphrodite, harmonia).
father(cadmus, semele).
mother(harmonia, semele).
father(zeus, dionysus).
mother(semele, dionysus).

% Rules

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

/* This one loops
ancestor(X,Y) :- parent(X,Y).
ancestor(X,Y) :- ancestor(X,Z), parent(Z,Y).
*/

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

sibling(X,Y) :- mother(M,X), mother(M,Y),
                X \== Y,
                father(F,X), father(F,Y).

halfsibling(X,Y) :- parent(Z,X), parent(Z,Y),
                    X \== Y,
                    \+ sibling(X,Y).

/* Example run:

   % prolog
   SICStus 3.7: Tue Sep 08 19:55:50 MET DST 1998
   Licensed to cs.rutgers.edu
   | ?- [family].
   {consulting family.pl...}
   {family.pl consulted, 10 msec 3464 bytes}

   yes
   | ?- ancestor(zeus,D).

   D = ares ? ;; 

   D = dionysus ? ;

   D = harmonia ? ;

   D = semele ? ;

   D = dionysus ? ;

   no
   | ?- ancestor(A,alice).

   A = victoria ? ;

   A = albert ? ;

   no
   | ?- sibling(X,Y).

   X = edward,
   Y = alice ? ;

   X = alice,
   Y = edward ? ;

   no
   | ?- halfsibling(X,Y).

   X = ares,
   Y = dionysus ? ;

   X = dionysus,
   Y = ares ? ;

   no
   | ?- halt.

*/