Macro module 

macro_rules! module {
    ($($entity:ty $(=> $behavior:expr)?),+ $(,)?) => { ... };
    (@build $module:expr; $entity:ty => $behavior:expr, $($rest:tt)*) => { ... };
    (@build $module:expr; $entity:ty, $($rest:tt)*) => { ... };
    (@build $module:expr; $entity:ty => $behavior:expr) => { ... };
    (@build $module:expr; $entity:ty) => { ... };
}