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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _update_phi(self):
 """Update `phi` using current `eta`."""

        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