in src/detail/field_registry.cpp [41:49]
void field_registry::save(const std::string &name, bool required,
const std::shared_ptr<field> &f) {
const auto was_saved =
_fields.insert(typename field_map::value_type(name, f)).second;
if (was_saved) {
_field_list.push_back(std::make_pair(escape_key(name), f));
_num_required_fields += required ? 1 : 0;
}
}