For #6 refactor to create and use enumerate_fib_suffix(n, suffix).
For #6 add d_rule, with tests (which record differences from D_G, previous wrong rule for D.
For #6 add c_rule, with tests (again records differences from C_G, previous wrong rule.
For #6 show that computed g_from_cd is OK up to n=7, has 1 error at n = 8 and 6 at n = 9.
For #6 add failing test for d_rule_2 applied to 'CDCD'.  I made a programming BLUNDER.