'Initialize the zeros, poles, gain system.'

def __init__(self, *system, **kwargs):

    if isinstance(system[0], LinearTimeInvariant):
        return
    super(ZerosPolesGain, self).__init__(**kwargs)
    self._zeros = None
    self._poles = None
    self._gain = None
    (self.zeros, self.poles, self.gain) = system
