Matt Hooper

I did put a basic use-case in the method doc but probably should have put it in the description. That example is negating a header filter to ignore requests with...

@apiraino Yes it can be done fairly easily like that too, although with `negate` its much more concise: ```rust fn is_old_api_user() -> impl Filter + Clone { warp::header::optional::("X-Api-v1").negate(|_| warp::reject()) }...

This reminds me of `Iterator`s [`flat_map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.flat_map) if that helps with naming.