when
What to do when 𝒳(s) is empty (§4.4.6.3). Zeros by default; §8.2.3's declared neutral value is what should replace this once it exists.
What to do when 𝒳(s) is empty (§4.4.6.3). Zeros by default; §8.2.3's declared neutral value is what should replace this once it exists.