Ah. And `child.attributeSignature + child.subtreeSignature` is still a numeric addition, but that's ok because order doesn't matter for those.

Regarding tag name, if we force a different tag ID, it still looks to me like we won't properly notice a difference in the subtree structure, because the tag ID is only used in `childSignature`, which is never incorporated into `subtreeSignature`. (So we'll notice a change in tag ID for direct descendants, but we might skip a whole subtree even though further down one of the tag IDs changed.)