Function custom_walk 

pub fn custom_walk<X, Y>(
    params: &WalkParams<X, Y>,
) -> Result<WalkPath, SimulationError>
where
    X: Copy + TryInto<Positive> + AddAssign + Display,
    Y: TryInto<Positive> + Display + Clone,

Built-in Custom (mean-reverting volatility) walk kernel: simulates the price path together with the volatility path that drove it.

This is the shared implementation behind the default WalkTypeAble::custom and WalkTypeAble::custom_with_vol methods, exposed publicly so custom walkers overriding one of them can compose or wrap the built-in dynamics instead of reimplementing them.

Errors

Same as WalkTypeAble::custom.