HashTableNT: support adding and removing elements from the namedtuple?

[ThomasWaldmann]

Think about whether we want such functionality in HashTableNT or if such stuff rather belongs elsewhere (like in the HashTableNT users code):

• dropping an element and loading an old ht that still has it is relatively straightforward
• adding an element and loading an old ht that does not yet have it is more difficult: would need a default value for the new element.

[ThomasWaldmann]

https://docs.python.org/3/library/collections.html#collections.namedtuple

• we could require giving a default for all (or new) elements in the namedtuple defaults list
• _field_defaults is the namedtuple attribute that stores the defaults