Computes 2nd derivatives of |complex power flow|**2 w.r.t. V.

def d2ASbr_dV2(dSbr_dVa, dSbr_dVm, Sbr, Cbr, Ybr, V, lam):
    """ Computes 2nd derivatives of |complex power flow|**2 w.r.t. V.
    """
    diaglam = spdiag(lam)
    diagSbr_conj = spdiag(conj(Sbr))

    Saa, Sav, Sva, Svv = d2Sbr_dV2(Cbr, Ybr, V, diagSbr_conj * lam)

    Haa = 2 * ( Saa + dSbr_dVa.T * diaglam * conj(dSbr_dVa) ).real()
    Hva = 2 * ( Sva + dSbr_dVm.T * diaglam * conj(dSbr_dVa) ).real()
    Hav = 2 * ( Sav + dSbr_dVa.T * diaglam * conj(dSbr_dVm) ).real()
    Hvv = 2 * ( Svv + dSbr_dVm.T * diaglam * conj(dSbr_dVm) ).real()

    return Haa, Hav, Hva, Hvv