Macro ops::impl_monoid_generics

macro_rules! impl_monoid_generics {
    (
        $name:ident[$($gen:tt)*] where [$($where:tt)*] =
            ([$ty:ty, $op:expr, $id:expr] [])
    ) => { ... };
    (
        $name:ident[$($gen:tt)*] where [$($where:tt)*] =
            ([$ty:ty, $op:expr, $id:expr] [$marker:ident])
    ) => { ... };
}