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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 _q_iteration(self, Q, Bpp_solver, Vm, Va, pq):
 """ Performs a Q iteration, updates Vm. """

        dVm = -Bpp_solver.solve(Q)

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

        return V, Vm, Va