Your definition of `ancestor` is one or two steps of parentage:

> ancestor(A, B) :- parent(A, B).
> ancestor(A, B) :-
parent(A, C),
parent(C, B).

I suspect you want one of those lines to appeal to the `ancestor` relation
to allow longer chains.

- Sam Caldwell

On Wed, Jan 31, 2018 at 1:53 PM, Kevin Forchione <lyss...@gmail.com> wrote:

> Walking through the datalog  tutorial I got the following transcript:
>
> Welcome to DrRacket, version 6.12 [3m].
> Language: datalog, with debugging; memory limit: 512 MB.
> > parent(john, douglas).
> > parent(john, douglas)?
> parent(john, douglas).
> > parent(john, evlyn)?
>
> > parent(bob, john).
> > parent(A, B)?
> parent(john, douglas).
> parent(bob, john).
> > parent(ebbon, bob).
> > parent(john, B)?
> parent(john, douglas).
> > parent(A, A)?
>
> > ancestor(A, B) :- parent(A, B).
> > ancestor(A, B) :-
> parent(A, C),
> parent(C, B).
> > ancestor(A, B)?
> ancestor(ebbon, bob).
> ancestor(bob, john).
> ancestor(john, douglas).
> ancestor(bob, douglas).
> ancestor(ebbon, john).
> >
>
> It seems that the correct answer should also include ancestor(ebbon,
> douglas). Am I doing something wrong?
>
> -Kevin
>
>
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