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What's the consensus on `sentinel` vs `None`? Assume no backwards compatibility concerns. Do you still use `None` for pretty much all “nothing” usages and use `sentinel` only when `None` doesn't cut it? Or are you going to default to `sentinel` for placeholders?

Not doing it yet, but I will likely only use sentinel when there needs to be a semantic difference between "empty" and "flag". None is still perfectly valid in most situations.