'Convert to `ZerosPolesGain` system, without copying.
Returns
sys: ZerosPolesGain
The `ZerosPolesGain` system. If the class is already an instance of
`ZerosPolesGain` then this instance is returned.'

def _as_zpk(self):

    if isinstance(self, ZerosPolesGain):
        return self
    else:
        return self.to_zpk()
