maybe-commit fails unconditionally

maybe-commit from the low-proposals package seems to always throw an
exception, claiming that there is no proposal, even when used inside
e.g. ensure-atomicity.

(current-proposal) => #f
(ensure-atomicity (current-proposal)) => #(#t #(#f) #(#f) #f)

(maybe-commit) => assertion-violation: no current proposal
(ensure-atomicity (maybe-commit)) => assertion-violation: no current proposal


-Roderic