Function rk4_step 

pub fn rk4_step<F>(f: &F, t: f64, y: &[f64], h: f64) -> Vec<f64>
where
    F: Fn(f64, &[f64]) -> Vec<f64>,

One classical RK4 step of y' = f(t, y) for a state of any dimension. f returns the derivative vector, which must have the same length as y.