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?


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