ing forward iterator. 
  Forward iterators are needed for backtracking where the iterator needs to be 
  saved and restored later. Generally speaking, Spirit is a backtracking parser. 
  The implication of this is that at some point, the iterator position will have 
  to be saved to allow the parser to backtrack to a previous point. Thus, for 
  backtracking to work, the framework requires at least a forward iterator.<br>
  <br>
  Some parsers might require more specialized iterators (bi-directional or even