First of all, like the light spells, it wants to temporarily change the attribute of an object. On my first attempt I tried to do it the way I wanted to do light: by calculating its contribution to defense on the fly. What finally broke down was the scheme procedure wasn't returning an integer like it "should" have back to the C code. Frankly I don't know what it was returning. But at that point I was already somewhat disgusted with the whole thing, because all I really want to do is add another set of dice to the
defense roll. Using an effect to exec the dice roll just feels wrong, like I'm
starting to use the proverbial hammer on everything in sight.

Second of all, it needs to be applied to the party but affect all the
members. I'd like the effect to expire at the same time for everybody in the
party. In the end, I didn't achieve this, although I sense I could have with a
disproportionate amount of extra effort.

And that, ladies and gentleman, is why a design is never done. If you add
enough features it will start to buckle. But one can always shore it up with
some extra pilings and keep on going, which is what I'll do.