It's disappointing to have to do this, but I think necessary because
various issue reports have shown it to be unsound (at least as far as
inference goes) and, at the very least, confusing. This patch brings us
back to the basic rules of QTT.
On the one hand, this makes the 1 multiplicity less useful, because it
means we can't flag arguments as being used exactly once which would be
useful for optimisation purposes as well as precision in the type. On
the other hand, it removes some complexity (and a hack) from
unification, and has the advantage of being correct! Also, I still
consider the 1 multiplicity an experiment.
We can still do interesting things like protocol state tracking, which
is my primary motivation at least.
Ideally, if the 1 multiplicity is going to be more generall useful,
we'll need some kind of way of doing multiplicity polymorphism in the
future. I don't think subtyping is the way (I've pretty much always come
to regret adding some form of subtyping).
Fixes#73 (and maybe some others).
Sort by proportion of bound variables used, which is likely to get us
the right answer quicker. The results are generated in batches of 16 (a
completely arbitrary choice) then sorted.
Rather than returning a complete list of results, return a pair of the
first result, and a continuation. The continuation explains how to
continue the search if the given result is deemed unacceptable (either
on encountering an error somewhere, or just if the caller wants the next
result).
This means we don't search needlessly if we're only looking for the
first result. Fixes#228
