Macro signature_by_type

macro_rules! signature_by_type {
    ($a1:ty , $($arg:ty),* => $_return:ty) => { ... };
    ( => $_return:ty) => { ... };
    ( $a1:ty => $_return:ty) => { ... };
    ( => ) => { ... };
    ( $a1:ty => ) => { ... };
    ($a1:ty , $($arg:ty),* => ) => { ... };
}