code: def _update_phi(self):
          
          etaprod = 1.0
          for w in range(N_NT - 1):
              self.phi[w] = etaprod * (1 - self.eta[w])
              etaprod *= self.eta[w]
          self.phi[N_NT - 1] = etaprod
docs: Update `phi` using current `eta`.