Function xapphire::prelude::solve_cg_precond

pub fn solve_cg_precond(
    a: &Array<f64, Ix2>, 
    b: &Array<f64, Ix1>, 
    x0: &Array<f64, Ix1>, 
    precond: &Array<f64, Ix2>, 
    eps: f64, 
    iter_max: usize
) -> Result<(Array<f64, Ix1>, usize, f64), &'static str>