mod amethyst {

    pub struct Builder;

    impl Builder {

        pub fn with_resource(self) -> Self {
            self
        }
    }
}

mod phythyst {

    pub trait PhythystBuilderExt: Sized {
        fn with_physics(self, physics: usize) -> Self;
    }

    impl PhythystBuilderExt for crate::amethyst::Builder {
        fn with_physics(self, _physics: usize) -> Self {
            println!("Here you can register stuff!");
            self
        }
    }

}


fn main() {
    use phythyst::PhythystBuilderExt;

    let _x = amethyst::Builder
        .with_resource()
        // voilá
        .with_physics(0);
}