The person who last pushed.

jj force pushes by default, but unlike git, the force push defaults to the "safe" variant (similar to git's --force-with-lease). So you won't accidently overwrite someone else's changes with your force push.

In jj, pulled commits are immutable by default. Commits that you author are mutable until you push them.

If I'm on a project with change AAA -> BBB, suppose my friend authors CCC and I author DDD.

So my history looks like:

    AAA  ---->  BBB  ---->  DDD
    (immutable) (immutable) (mutable)

And my friend's has:

    AAA  ---->  BBB  ---->  CCC
    (immutable) (immutable) (mutable)

But then my friend pushes their change first. So when I pull, I'll have:

    AAA -> BBB -> CCC (immutable)
            \
             '-> DDD (mutable)

Just like git, I then rebase my commit by saying `jj next` (mnemonic: "move this commit to its next descendant") to get

    AAA -> BBB -> CCC (immutable) -> DDD (mutable)