Macro compose 

macro_rules! compose {
    (@ call $builder:ident; $key:ident : type $ty:ty) => { ... };
    (@ call $builder:ident; $key:ident < $($g:ty),+ > : $value:expr) => { ... };
    (@ call $builder:ident; $key:ident : $value:expr) => { ... };
    (@ entries $builder:ident; $key:ident : type $ty:ty $(, $($rest:tt)*)?) => { ... };
    (
        @ entries $builder:ident; $key:ident < $($g:ty),+ > : $value:expr $(,
        $($rest:tt)*)?
    ) => { ... };
    (@ entries $builder:ident; $key:ident : $value:expr $(, $($rest:tt)*)?) => { ... };
    (@ entries $builder:ident;) => { ... };
    ($variant_id:ident : $($rest:tt)*) => { ... };
}