Why are Expander's fields private?

[mindreader]

I want to use a slightly different expansion method, the default, but with allow_undelimited_name: false, but I can't see a way to do it with the public api available.

[keith-hall]

I would guess that the idea is to only expose things we know will be used, because once something is public we need to be more careful when making changes etc. and support various previously unthought of interactions.

I'd say open a PR, and then we can get more of an idea of how much maintenance burden it could add.