Function assemble_kkt 

pub fn assemble_kkt(
    a: &CsMat<f64>,
    b: &[f64],
    l: &CsMat<f64>,
    q_bus: &[f64],
    theta_f_inv: &[f64],
    theta_g_inv: &[f64],
    r: usize,
    want_l_eff: bool,
) -> Result<KktOperators>

Assemble the reduced KKT operators from the case factors and the caller-supplied positive interior point diagonals.

theta_f_inv (length m) and theta_g_inv (length n) are the barrier weights Θf⁻¹, Θg⁻¹; q_bus (length n) is the bus cost diagonal; r is the slack bus index.