Sometimes we swap the arguments, to reduce code duplication, but we need
to remember we've done that since (1 x : a) -> b is valid for an
argument of type (x : a) -> b, but not vice versa (that is, we have a
teensy bit of subtying to deal with, for convenience...).
This fix seems a bit ugly, but we do at least now propagate the
information. Fixes#82.
