def _update_phi(self):
etaprod = <NUM_LIT:1.0><EOL>for w in range(N_NT - <NUM_LIT:1>):<EOL><INDENT>self.phi[w] = etaprod * (<NUM_LIT:1> - self.eta[w])<EOL>etaprod *= self.eta[w]<EOL><DEDENT>self.phi[N_NT - <NUM_LIT:1>] = etaprod<EOL>
Update `phi` using current `eta`.