31.1 Nulling versus nulled

Currently we call a zero-length instance (aka tree node) either a nulling instance or a nulled instance. The use of “nulling” is for historic reasons and arguably is confusing. The symbol of a nulling instance is not necessarily a nulling symbol — it might be a nullable symbol. Usage of the term “nulled” is less confusing. At this time, we continue to allow zero-length instances to be called nulling instances because that terminology is embedded in a lot of code and documents.