@witty-crayon-22786 saw it and makes sense. It does look like the gil is released (http://cffi.readthedocs.io/en/latest/ref.html#conversions note [2]). If so / barring real perf issues, I definitely favor making rules look like 1st class values that can be passed to other rule functions, but I may be wrong in guessing this would be a simpler / more clear model for end-user rule writers; so not a super-strong objection if the coroutine syntax must stay.