On the play-level, we have
serial: 1 to allow us to run the whole play one host at a time. But I haven't found a simple way to do this on a single task. This is especially relevant, if the task in question doesn't perform proper locking (for whatever reason).
One obvious answer is to put the task in its own play. But that doesn't help with roles. (Having to put
serial: 1 on the play using the role isn't really intuitive.)