Lecture 1
Lecture 2
Lecture 3
Lecture 4
Lecture 5


The predicate grandparent_of/2 could be defined as follows:

grandparent_of(X,Y) :- parent_of(X,Z), 
The predicate greatgrandparent_of/2 could be defined as follows:
greatgrandparent_of(X,Y) :- parent_of(X,Z), 
And the predicate greatgreatgrandparent_of/2 could be defined analogously by simply adding one more parent_of step to the definition of greatgrandparent/2. However, ancestor_of/2 cannot be defined in this way, as we don't know how many 'parents we have to go back' to find an ancestor. Anybody who is an arbitrary number of parent relations back is an ancestor.

So, how can we express what ancestor_of means? Well, obviously if I am the parent of somebody, I also belong to his ancestors. Furthermore, if I am the parent of somebody and that somebody is an ancestor of a third person, I am also an ancestor of this third person.


Back to the exercise.