This. The whole argument only holds because you can choose `isspace` to return false for '\0'. You should be able to choose whichever sentinel fits to the requi

This. The whole argument only holds because you can choose `isspace` to return false for '\0'. You should be able to choose whichever sentinel fits to the required invariant, and C's global string "sentinel" is less useful in that regard. If you need a sentinel you should be able to push one beforehand anyway.