complete proof about flush vs flushness equivalence
complete proof about flush vs flushness equivalence

forall h, legal h -> hlegal (drop_flush h).

combined with the earlier proof that eager and lazy impls are legal,
this means those two impls are also Haogang-style legal (hlegal).

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl