> generates recursive descent
          parser which require backtracking parsers by design. For this reason we
          need to provide at least forward iterators to any of <span class="emphasis"><em>Spirit.Qi</em></span>'s
          API functions. This is not an absolute requirement though. In the future,
          we shall see more deterministic parsers that require no more than 1 character
          (token) of lookahead. Such parsers allow us to use input iterators such
          as the <code class="co