multi-predicate learning not working

[andrewcropper]

Metagol should support multi-predicate learning (and used to). For instance, it should be able to learn grandmother/2 and grandfather/2 at the same time, such as:

grandfather(A,B):-father(A,C),parent_1(C,B).
grandmother(A,B):-mother(A,C),parent_1(C,B).
parent_1(A,B):-mother(A,B).
parent_1(A,B):-father(A,B).

The problem is that I do not add all target symbols to the predicate signature. I quickly tried to but it still did not work satisfactorily.