two small suggestions: change .saving() to .mutable() and .rem() to .remove() (or filter(), reversing the semantics, since that is already in python) (every other method is a full word).

also, maybe .mutable() would be better as a flag in the constructor. it makes no sense to use it in a chain (in fact, that could be confusing) and is the kind of thing you should probably fix on construction rather than changing later.

ps. it's not clear to me that this is better than list comprehensions, which already do much of what you have.