This is quite fiddly as it the blocks might be in different contexts so
we need to keep track of which variables correspond in the scrutinees of
the blocks. Once that's done, check the terms at the leaves convert,
then check the corresponding variables convert.
This may not be perfect yet, because we only look at case scrutinees to
find correspondence. It might also be a bit slower than it could be, but
at least these checks are quite rare.
Fixes#208 and maybe some others?
