!= package:promises

Fulfill a promise. Each promise should only be fulfilled once.

>>> runLazy_ $ \p -> p != "good"
"good"

>>> runLazy_ $ \p -> do q <- promise_; p != "yay! " ++ demand q; q != "it works."
"yay! it works."

>>> runLazy_ $ \p -> return ()
*** Exception: BrokenPromise

>>> runLazy (\p -> return ()) "default"
"default"