For #8 add index_from_fibword and make explicit what's missing from n=6.  Have identified the bug, can now fix it.
For #8 track down what's going wrong - there's an error in d_rule.
For #8 fix the d_rule BLUNDER (easy) and change wrong tests to agree with good code.
For #8 add detect_collisions, args_from_collisions, and show_collisions(9) - two collisions.