Kinda off-topic, but I also have to say "appendable" is a terrible choice. It would mean that those objects (or variables, or whatever) support efficient appen

Kinda off-topic, but I also have to say "appendable" is a terrible choice. It would mean that those objects (or variables, or whatever) support efficient appending operation, which is usually not the case. (Sorry, I couldn't get over the fact that appending a character to a string (or a list) is O(N) operation in Haskell.) Now, if you named them "prependable", then maybe...