Function compose 

pub fn compose<S, B, A, L1, L2>(outer: L1, inner: L2) -> Composed<L1, L2, B>
where
    L1: Lens<S, B>,
    L2: Lens<B, A>,
    B: Clone,

Compose two lenses: outer then inner.

The resulting lens focuses on inner’s target through outer’s target.