Function carry_assets 

fn carry_assets<S: Storage, T: Storage>(
    from: &S,
    to: &Container<T>,
    at: Rev,
) -> Result<(), PrefixFailure>

Copy the artwork the carried revs reference from one container’s assets/ to another’s. Content-addressed, so the file’s name in the new container is the name it had in the old one and nothing has to be rewritten.

The revs are read back rather than the whole directory copied: what a save-as owes is the payloads its own history points at, and a payload nothing past the cut references is not part of what was asked for.