s it slow down the program due to the virtual function indirect 
  call, it is also an opaque wall where no metaprogram can get past. It kills 
  all meta-information beyond the virtual function call. Worse, the virtual function 
  cannot be templated. Which means that its arguments have to be tied to a actual 
  types. Many problems stem from this limitation. <br>
  <br>
  While Spirit is a currently classified as a non-deterministic recursive-descent 
  parser, Doug Gregor first noted that other parsing t