/* this example is adapted from Clocksin and Mellish,
 "Programming in Prolog", Springer-Verlag, 1981. */

female(victoria).
female(alice).
male(albert).
male(edward).

/* Child has parents Mother and Father */

has_parents(edward, victoria, albert).
has_parents(alice, victoria, albert).

/* Has_sis has sister Is_sis */

has_sister(Has_sis, Is_sis) :- female(Is_sis),
                    has_parents(Has_sis, M, F),
                    has_parents(Is_sis, M, F).