Non MGF calls in update algorithm

[georgematheos]

I need to do a pass over the update algorithm to make sure that I understand how non MGF calls (in particular, calls to :abstract) are handled!

Currently, I have modified lookup_or_generate_during_world_update! by adding

    if no_val && !is_mgf_call(call)
        @assert addr(call) == _get_abstract_addr
        generate_value!(world, call, EmptyChoiceMap())
        return get_val(world, call)
    end

to silence a bug which was occurring when processing a new :abstract call in the GMM example.

I should return to this and understand whether this is the right solution (and if so convince myself that it won't have unintended side effects), or find a better one.

[georgematheos]

Update:
This is the commit where I implemented a fix for this: bb8657b#commitcomment-55919543

In the end, I changed the code snippet I mentioned above to

 if no_val || !is_new_lookup || !already_updated(world, call)
         update_or_generate!(world, call)
         if is_mgf_call(call)
             update_or_generate!(world, call)
         else
             # I have this assertion since I have in mind that this is the only place
             # where this could happen, but am not 100% sure, and I want to be alerted if
             # this turns out to be false.
             @assert no_val && addr(call) == _get_abstract_addr

             generate_value!(world, call, EmptyChoiceMap())
         end
     end