For #8 add cd_trace, but results don't agree with tests/issue6.py.
For #8 add c_rule test that should fail.
For #8 fix two c_count errors - both BLUNDERs. Still not agreeing with tests/issue6.py.
For #8 add properties Index.c and Index.d.
For #8 make Index hashable and use as key in cd_trace.