from permchain import Pregel, channels

value = channels.LastValue[str]("value")

grow_value = (
    Pregel.subscribe_to(value)
    | (lambda x: x + x)
    | Pregel.send_to({value: lambda x: x if len(x) < 10 else None})
)

pubsub = Pregel(grow_value, input=value, output=value)

assert pubsub.invoke("a") == "aaaaaaaa"