katmagic/curve25519-donna — comment on commit aa8e3abd2bbc1d669fd0dfa9b55a6988e96f12c3
File: curve25519-donna.c line 458
By agl:

This isn't clearly true. The loop can decrease a limb by > 1 in the general case. It might be that the preconditions to this function exclude that possibility, but the preconditions aren't documented ;) (Because the 32-bit version was only a demo; I figured that people would use the much faster djb code on 32-bit systems.)

So it may be worth updating this comment in the future.