or unused_type is a no-op. Now, you can imagine why <a href="http://boost-spirit.com" target="_top">Spirit</a> is fast! The parsers are so
          simple and the generated code is as efficient as a hand rolled loop. All
          these parser compositions and recursive parse invocations are extensively
          inlined by a modern C++ compiler. In the end, you get a tight loop when
          you use the kleene. No more excess baggage. If the attribute is unused,
          then there is no code generated 