Liveness of fallback tags is a forward-propagagation problem.

Liveness of fallback tags is a forward-propagagation problem.

re2c used to merge all possible fallback tags and backward-propagate
them from states that have default transitions. This is an
overapproximation; some tags are marked alive while they are not.

Liveness of fallback tags should be forward-propagated, because
this is the only way to track paths that really need these tags
(that is, paths that start in final state and end with default
transition *without passing through another final state*).

Note that sometimes we can get to the same default transition
following two different paths, one path may passing through final
state; the other is not. One path cares for fallback tags; the
other doesn't. We have no way to find out if we start bacward
prpagation of fallback tags from default transition.