Looks like you're actually computing the full product. The only optimization you're doing is forgetting the low order bits as soon as you're done with them. The

Looks like you're actually computing the full product. The only optimization you're doing is forgetting the low order bits as soon as you're done with them. The author is asking to keep them, which sounds reasonable. You initially claimed they were "never computed in the first place", and we're asking you to back up that claim if you still think it's true.