Function cg_solve_masked 

fn cg_solve_masked<F, Op>(
    b: &[f64],
    _mask: &[f64],
    max_iter: usize,
    tol: f64,
    callback: &mut F,
    apply_ata: Op,
) -> Vec<f64>
where
    F: FnMut(usize, usize),
    Op: Fn(&[f64]) -> Vec<f64>,

Generic CG solver for masked normal equations