null fails only while no attempt has claimed the executor. A claim fails
only while it is still the latest and no provider reference is recorded.
OptionalunlessFails only while no RouteFusion ticket is recorded, so a payout attempt cannot fail a ticket another attempt recorded.
Conditions a send attempt puts on its FAILED mark, so it cannot fail a transfer another attempt claimed after it, ordered by when the claims commit. The mark applies only while every condition given holds.