Support for Execution monad

Commented on this topic (at the same time you wrote this post) in reply to your earlier points above. Oops!

TLDR: Verifiable could be extended to support the return type. I think the one call to either ok or facts or expectations is reasonable, as they form a power hierarcy iirc.

Anyway, regardless of this issue, I think the extra functionality is looking pretty awesome. :)