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)