impl<'r> Ecs<'r> {
    pub fn add_entity<A: 'static>(&'r mut self) {
        let index: usize = {
            let p: Option<usize> = {
                self.world // immutable borrow here
                    .iter()
                    .position(|store| {
                        store.types().len() == 1 && store.types().all(|ty| *ty == TypeId::of::<A>())
                    })
            };
            let index = p.unwrap_or(self.world.len());
            if p.is_none() {
                let mut store = VecTypeStore::new();
                store.insert(Vec::<A>::new());
                self.world.push(store); // mutable borrow here
            }
            index
        };
    }