What if mutability was part type's feature? And I don't mean that type can be mutable or immutable, I mean it can be both at different times.
And what mutability even means for series types? Does it mean that series is fixed in size and only its values can change, or does it mean it can also expand or shrink freely?
Well @recoil has a solution - each series type has an attribute, #immutable, #mutable and #flexible. Mutable series can change content but cannot change size,flexible series can both



