Attempt rules not preserving definedness when configuration is concrete

[goodlyrottenapple]

As a first attempt in dealing with partial functions in configurations, we could try to unify with rules introducing partial function symbols, if all the arguments are fully concrete. Then we would simply send these off to the LLVM back-end. However, not sure how the LLVM back-end would deal with partial functions which are in fact undefined for the inputs. I seem to remember it just crashes and takes the server down with it in such cases?