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.
