Rec.v: prove [word_updN_equiv]
Rec.v: prove [word_updN_equiv]

what a ridiculously complicated proof!  an unfortunate combination of
complex dependent types and an actually tricky operation (using split
and combine operations to splice an update into the middle of a block).

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