Solved problem 65.
Pr66: Passes the tests, but is slow.
Pr66 runs in linear time, but d=61 presents a problem.
Solved problem 66.
Pr68: Works on the test case, but short-sighted.

I need to enforce the rule that out of the (2*SIDES) numbers, SIDES
numbers appear exactly once and the other SIDES numbers appear exactly
twice.
Solved pr68, pr69, pr70.
Solved pr71.