Macro generics::parse

macro_rules! parse {
    (
        $callback:path { $($callback_args:tt)* } < $($token:tt)*
    ) => { ... };
    (
        $callback:path { $($callback_args:tt)* } $(($($tuple:tt)*))? where $($token:tt)*
    ) => { ... };
    (
        $callback:path { $($callback_args:tt)* } $($token:tt)*
    ) => { ... };
}