code: def _eta_from_phi(self):
          
          self.eta = scipy.ndarray(N_NT - 1, dtype='float')
          etaprod = 1.0
          for w in range(N_NT - 1):
              self.eta[w] = 1.0 - self.phi[w] / etaprod
              etaprod *= self.eta[w]
          _checkParam('eta', self.eta, self.PARAMLIMITS, self.PARAMTYPES)
docs: Update `eta` using current `phi`.