122: initial commit
122: initial test build (buggy)
122: bug fix

Turns out to be a stupid typo.
122: optimize

Working now, but got stuck on 8. I just realized that we are using
IDDFS, so there is no point of finding smaller depth once a depth for a
certain exponent has shown up. Hope this optimization at least help a
bit.
122: do recursion count

depth=8 stuck for about 10 minutes. But here is the recursion count for
depth 2--7:

depth: 2; recursion count: 3
depth: 3; recursion count: 13
depth: 4; recursion count: 59
depth: 5; recursion count: 355
depth: 6; recursion count: 2865
depth: 7; recursion count: 29424

The number of recursions should not blow up, at least for 8. So it
suggests that maybe the problem is stack size?
122: fix overflow error

Turns out the problem is the size of the _trace array... I wouldn't
have discovered this unless I added a recursion count variable (and the
alignment also favors me)
122: testing better than expected

Out of the 200 exponents, only 191 cannot be generated in 10 steps; and
obviously it will be generated given one additional step.
122: finish