Realized that none of my sub-expression tests were actually testing anything.  Fixed that, and then fixed another bug in the subexpression evaluation algorithm that the now fixed tests revealed.
Fix bug in isinstancemethod when used with a subclass where the method in question is defined on some parent class.
Cleaned up and improved logic for indexing and slicing of compound model classes *and* instances, by name, by index, or some combination thereof.  Added a few basic tests of the capability.