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

def d2AIbr_dV2(dIbr_dVa, dIbr_dVm, Ibr, Ybr, V, lam):
    """ Computes 2nd derivatives of |complex current|**2 w.r.t. V.
    """
    diaglam = spdiag(lam)
    diagIbr_conj = spdiag(conj(Ibr))

    Iaa, Iav, Iva, Ivv = d2Ibr_dV2(Ybr, V, diagIbr_conj * lam)

    Haa = 2 * ( Iaa + dIbr_dVa.T * diaglam * conj(dIbr_dVa) ).real()
    Hva = 2 * ( Iva + dIbr_dVm.T * diaglam * conj(dIbr_dVa) ).real()
    Hav = 2 * ( Iav + dIbr_dVa.T * diaglam * conj(dIbr_dVm) ).real()
    Hvv = 2 * ( Ivv + dIbr_dVm.T * diaglam * conj(dIbr_dVm) ).real()

    return Haa, Hav, Hva, Hvv