t this is the way <a href="http://boost-spirit.com" target="_top">Spirit</a>
        has been used since its invention.
      </p>
<p>
        However, this separation has both practical and theoretical basis, and proves
        to be very useful in practical applications. In 1956, Noam Chomsky defined
        the "Chomsky Hierarchy" of grammars:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            Type 0: Unrestricted grammars (e.g