It's worth delaying in case doing some more work (and some more
interface resolution) can make the type more concrete. This makes
test linear011 work more smoothly, and will help with this sort of
program in general.
A better way, later, would be to try elaborating and delay only if the
type is not yet known. We have the facilities, but it's too fiddly for
me to want to implement it right now...
We need to check below top level too, since there could be holes that
we're happy to resolve by searching. The linearity test added
illustrates a place where this is needed.
Doing this is awkward, for a number of reasons, including 'pure' not
being linear in its argument - there's no guarantee it'll be used
linearly after all - and lack of multiplicity polymorphism. Fortunately
the user doesn't have to know about the ugliness underneath!
We can look at ways to make it less horrible later :). For now, this is
starting to look like something that allows us to write linear protocols
without too much extra machinery on top of IO.
