Macro join_impl::define_group_determiner

macro_rules! define_group_determiner {
    ($combinator: expr => $($tokens: expr),+ => $length: expr => $validate_parsed: expr) => { ... };
    ($combinator: expr => $($tokens: expr),+ => $length: expr) => { ... };
}

Creates GroupDeterminer with given (Combinator => tokens => length => ?Check parsed tokens? (optional bool))

Example:

use join_impl::chain::group::Combinator;
use join_impl::define_group_determiner;
use syn::Token;

let then_determiner = define_group_determiner!(Combinator::Then => Token![->] => 2); // last param is optional, `true` by default