much improved coverage and a few minor fixes in fixplate

Including coverage for essentially everything except for the morphisms
and a few odds and ends.

Fixes a handful of methods which did not work as advertised, and a couple of
type class instances which didn't seem to be usable as typed.

Note: a couple of tests are failing because I could not determine what the correct result should be.
I take that back, it looks like `holes2` is non-recursive.