Pm pln

Basic infrastructure for implementation of Attentional Focus filtering inside the the pattern matcher.the preliminary implementation of the callbacks could be changed later. one can use the specific implementation by invoking **cog-bind-pln**  from scheme.
Oh . I see .. this was crashing because you were returning the wrong type.  The incoming set is not a vector of handles, its a vector of LinkPtr.  Als, the LinkPtr of the Handle::UNDEFINED is null (its a null pointer) so it crashed because you were returning a null pointer.

Anyway, you don't need to perform this check at all, you only need to do the if-test below.