code: def d2Ibr_dV2(self, Ybr, V, lam):
          
          nb = len(V)
          ib = range(nb)
          diaginvVm = csr_matrix((ones(nb) / abs(V), (ib, ib)))
  
          Haa = csr_matrix((-(Ybr.T * lam) / V, (ib, ib)))
          Hva = -1j * Haa * diaginvVm
          Hav = Hva
          Hvv = csr_matrix((nb, nb))
  
          return Haa, Hav, Hva, Hvv
docs: Based on d2Ibr_dV2.m from MATPOWER by Ray Zimmerman, developed
          at PSERC Cornell. See U{http://www.pserc.cornell.edu/matpower/} for
          more information.
  
          @rtype: tuple
          @return: The 2nd derivatives of complex branch current w.r.t. voltage.