Maybe the following makes it more transparent to you:

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def get_or_create(actor, parm_name):
    """Helper function to get or create a parameter"""
    p = actor.Parameter(parm_name)
    if not p:
        p = actor.CreateValueParameter(parm_name)
    return p


SCENE = poser.Scene()


def init(actorname, framenr_start=0, modulo_value=360):
    def __callback__(parm, value):
        actor = parm.Actor()
        mod = actor.Parameter("Modulo Value").Value()
        actor.Parameter("Result").SetValue(value % mod)
        return value

    actor = SCENE.Actor(actorname)
    # Make sure the following parameters exist
    get_or_create(actor, "Frame Counter").SetValue(framenr_start)
    get_or_create(actor, "Modulo Value").SetValue(modulo_value)
    get_or_create(actor, "Result")

    # Set callback
    actor.Parameter("Frame Counter").SetUpdateCallback(__callback__)


# For demo tart with the currently selected actor in this case.
init(SCENE.CurrentActor().Name())

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