Error of logic?

Question

I have the following code:

mãe(ana, eva).
mãe(eva, noé).
mãe(bia, raí).
mãe(bia, clô).
mãe(bia, ary).
mãe(lia, gal).
pai(ivo, eva).
pai(raí, noé).
pai(gil, raí).
pai(gil, clô).
pai(gil, ary).
pai(ary, gal).
avô(X, Y) :- pai(X, Z), pai(Z, Y); pai(X, Z), mãe(Z, Y).

When making the consultation:

avô(X, Y).

SWI-Prolog returns to me:

X = gil,
Y = noé ;
X = gil,
Y = gal ;
X = ivo,
Y = noé ;
false.

I thought it should return 'true', but I didn’t understand if I made a logic error because I can’t identify it, I would like to know the reason to return 'false' (I’m quite beginner). Thank you.

Answer 1

There is no error in your code. Prolog is returning all pairs (X,Y) that satisfy the query avo(X,Y).

He would return true if Voce asked a question where both the "variables" X and Y "have some value":

avo(gil, noe)

for example.