/*  Filename:  ancestor2.pro

    Author:  Br. David Carlson

    Date:  December 6, 1999

    Revised:  March 29, 2000  Added a sibling and a child predicate.

    This program creates a small set of facts and rules on who is the ancestor
    of whom.  The user types the desired goal interactively when the program
    is run.
*/

child(C, P) :- parent(P, C).

/* You might not think of the A not equal to B condition.  So, I did not
   expect that you would include it.  However, without that, the program
   would say that john is a sibling of john, etc.
*/
sibling(A, B) :- parent(P, A), parent(P, B), A \== B.

/*  Note that ancestor(A, B) means that A is an ancestor of B. */
ancestor(bob, susan).
ancestor(A, X) :- parent(A, X).
ancestor(A, X) :- parent(A, C), ancestor(C, X).

/* Note that parent(P, C) means that P is a parent of C. */
parent(fred, sally).
parent(tina, sally).
parent(sally, john).
parent(sally, diane).
parent(sam, bill).