Function stationary 

fn stationary(jtj: &[Vec<f64>], gradient: &[f64], norm: f64) -> bool

Converged in the gradient (GRADIENT_TOLERANCE): at a minimum with a non-zero residual — every overdetermined fit — the residual is orthogonal to the Jacobian’s columns. A column that does not move the residual at all has nothing to say and is skipped rather than divided by.