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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def set_signal_type(self, sig_type):
 """Set the signal type of interest. Sets the signal type for which the SNR is calculated. This means inspiral, merger, and/or ringdown. Args: sig_type (str or list of str):
 Signal type desired by user. Choices are `ins`, `mrg`, `rd`, `all` for circular waveforms created with PhenomD. If eccentric waveforms are used, must be `all`. """

        if isinstance(sig_type, str):
            sig_type = [sig_type]
        self.snr_input.signal_type = sig_type
        return