code: def _get_total_sigma(self, imt, mag):
          
          C = self.SIGMA[imt]
          if mag <= self.magnitude_limits[0]:
              # The CENA constant model is always returned here
              return C[self.tau_keys[0]]
          elif mag > self.magnitude_limits[-1]:
              return C[self.tau_keys[-1]]
          else:
              # Needs interpolation
              for i in range(len(self.tau_keys) - 1):
                  l_m = self.magnitude_limits[i]
                  u_m = self.magnitude_limits[i + 1]
                  if mag > l_m and mag <= u_m:
                      return ITPL(mag,
                                  C[self.tau_keys[i + 1]],
                                  C[self.tau_keys[i]],
                                  l_m,
                                  u_m - l_m)
docs: Returns the estimated total standard deviation for a given intensity
          measure type and magnitude