jub:by is a variant of jab:by, which applies a gate to a value at a certain key. It crashes if it's not there, and also does not allow it to be deleted. This variant allows a create, update, or delete operation with one gate call and one tree traversal. The gate passed to jub takes a unit and returns a unit. The unit it is passed is ~ if the kv pair does not exist, and `val if it does. The unit returned causes a deletion if it is ~, and inserts otherwise.

urbit/urbit#6782 is the corresponding PR with the hoon.

I added a jet for jib, referenced in the PR by amsden these pair of PR's would replace. I haven't written the hoon for it yet. Maybe we don't want to add it, as jub is exhaustive and would only be an ergonomic addition.

I'm not sure if I have the reference hygiene right. Notably regarding the transfer/retain semantics.