Function null_tangent 

pub fn null_tangent(
    jacobian: &[Vec<f64>],
    gradient: &[Vector],
    tol: Tolerances,
) -> Option<Vector>

The unit space tangent of a condition’s curve from its Jacobian (the n − 1 rows of its equations) and its position gradient: the Jacobian’s null vector carried into space. What Condition::tangent answers by default. None where the Jacobian has full rank or the tangent vanishes.