Macro join_impl::define_tokens_checker

macro_rules! define_tokens_checker {
    ($token1: expr, $token2:expr, $token3:expr) => { ... };
    ($token1: expr, $token2:expr) => { ... };
    ($token: expr) => { ... };
    ($($token: expr),+) => { ... };
}

Creates function which checks if ParseStream next values are provided tokens.