Method setFlag

tests/catch.hpp:7103–7105

        explicit BoundFlagLambda( L const &lambda ) : m_lambda( lambda ) {}

        auto setFlag( bool flag ) -> ParserResult override {
            return LambdaInvoker<typename UnaryLambdaTraits<L>::ReturnType>::invoke( m_lambda, flag );
        }
    };

    enum class Optionality { Optional, Required };