:- add_rule(
father
    if parent(X, Y) and male(X) then add(father(X, Y))). 

:- add_rule(
grandfather
    if father(X, Y) and father(Y, Z) then add(grandfather(X, Z))). 

:- add(parent(abe, bob)).
:- add(male(abe)).
:- add(parent(bob, dan)).
:- add(male(bob)).