code: def sigma(htilde, psd = None, low_frequency_cutoff=None,
          high_frequency_cutoff=None):
      
      return sqrt(sigmasq(htilde, psd, low_frequency_cutoff, high_frequency_cutoff))
docs: Return the sigma of the waveform. See sigmasq for more details.
  
      Parameters
      ----------
      htilde : TimeSeries or FrequencySeries
          The input vector containing a waveform.
      psd : {None, FrequencySeries}, optional
          The psd used to weight the accumulated power.
      low_frequency_cutoff : {None, float}, optional
          The frequency to begin considering waveform power.
      high_frequency_cutoff : {None, float}, optional
          The frequency to stop considering waveform power.
  
      Returns
      -------
      sigmasq: float