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.
Btw, I think the above test correctly adds 1 to the leaves, which is the desired behavior based on the types.