However, getting the tmpl::pin, tmpl::parent, and tmpl::bind calls right can be extremely frustrating with little help as to what is going on. Let's introduce an error by pinning tmpl::_1:
using variables_tags_from_single_tags = tmpl::filter<
The result is comparing all values in extracted_from_variables to themselves. To find this out, replace tmpl::filter and tmpl::found with tmpl::transform, and the metafunction std::is_same to make_list. You will then get back a "backtrace" of what the algorithm did, which is invaluable for getting the tmpl::pin and tmpl::parent right. That is,