Comefrom could be usefull to implement aspect oriented semantics.

Invisible asserts are also possible for debugging purposes, that don't pollute the codebase. They also don't self-document the code but (and I might be mistaken) PHP can't just optimize some statements out so as to not have them eat cycles like compiled langs.

Both of these could sit in separate modules and be coupled in or not depending on weather one debugs or deploys.