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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 _p_iteration(self, P, Bp_solver, Vm, Va, pvpq):
 """ Performs a P iteration, updates Va. """

        dVa = -Bp_solver.solve(P)

        # Update voltage.
        Va[pvpq] = Va[pvpq] + dVa
        V = Vm * exp(1j * Va)

        return V, Vm, Va