code: def c_mvar(self):
          
          if self.b == -4:
              return 1.0
          elif self.b == -3:
              return 0.0
          elif self.b == -2:
              return -1.0
          elif self.b == -1:
              return -2.0
          elif self.b == 0:
              return -3.0
docs: return tau exponent "c" for noise type.
              MVAR = prefactor * h_a * tau^c