Replace

  at = root

with

  at = next[root]

and you're done. Once you understand the forward transform, you understand that root is the LAST byte and next[root] is the FIRST byte.

All the fixes in BUGFIX-66 are trivial if you understand what the code is doing, that's part of the game concept. Also, if you submit broken code, it gives you a useful hint to direct your attention to the bug.

Similarly, the fix for the forward transform is to add

  i1, i2 = x[i1], x[i2]

at the top of the less() function.