Macro world 

macro_rules! world {
    (
        // the name of the struct being defined
        $struct_name:ident, // the types of entities which can exist in the world
        $( $entity:ty ),* $(,)? // optional trailing comma
        ;// semi colon separator between lists
        // the traits which are query-able over all types in the world
        $( $trait_name:ident ),* $(,)? // optional trailing comma
    ) => { ... };
}