code: def _q_iteration(self, Q, Bpp_solver, Vm, Va, pq):
          
          dVm = -Bpp_solver.solve(Q)
  
          # Update voltage.
          Vm[pq] = Vm[pq] + dVm
          V = Vm * exp(1j * Va)
  
          return V, Vm, Va
docs: Performs a Q iteration, updates Vm.