Function then 

pub fn then<A, B, O>(
    operation: impl Operation<A> + 'static,
    next: impl Fn(A) -> O + Send + Sync + 'static,
) -> impl Operation<B>
where
    A: 'static,
    B: 'static,
    O: Operation<B> + 'static,

Chains the output of an Operation with the provided function to build a new Operation.