This starts the work on #108 by implementing
Z as well as
dropZ, and porting much of the associated theory.
I have not ported everything, e.g. the interactions with
mask are missing. Also there are more operations that would need a Z-based version, such as
sublist_alter. There are also some tactics that will not handle these new operations and so might need adjustments or Z-based tactics:
Still I think this is a useful start that can serve as a foundation for later extensions. :)
Blocked on !441 (merged).