For #9 refactor to create cd_trace_factory.
For #9 add stub test file issue9.py.
For #9 move *both* indices in d_rule, reduces errors in n=9 to 1 (from 2).
For #9 document only remaining collision in n=9.
For #9 anchor the value of cd_trace(5).
For #9 remove collisions in n=9.  Works also in n=10 and n=11.