I would like to easily retrieve the grouping fields after grouping by multiple fields.

[prehonor]

boolinq::from(top10)
    .groupBy([](const T& holder) {
        return std::make_tuple(holder.code, holder.date);  // something like this way
        // return holder.code + to_iso_extended_string(holder.date);  // Without using string concatenation methods
    })
    .select([](auto p){
        p.first.code;
        p.first.date
    });