rchy. We have a grammar that encapsulates the whole calculator. The 
  start rule is a plain rule that holds the set of subrules. The subrules in turn 
  defines the actual details of the grammar.</p>
<table width="80%" border="0" align="center">
  <tr> 
    <td class="note_box"><img src="theme/lens.gif" width="15" height="16"><b> 
      Template instantiation depth</b> <br> <br>
      Spirit pushes the C++ compiler hard. Current C++ compilers cannot handle 
      very complex heavily nested expressions ver