      We will use a setup, where we want the <span class="emphasis"><em>Spirit.Lex</em></span>
          library to invoke a given function after any of of the generated tokens
          is recognized. For this reason we need to implement a functor taking at
          least the generated token as an argument and returning a boolean value
          allowing to stop the tokenization process. The default token type used
          in this example carries a token value of the type <a href="../../../../../../../l