From: gah4@u.washington.edu   
      
   (our moderator wrote)   
      
   > [At the very least, you'd need some rules that don't have nonterminals   
   > on the right side to make it possible to break loops. -John]   
      
   So do it by back propagation.   
      
   Mark all rules that have a terminal on the right side.   
      
   Mark all rules that have a rule that has a terminal on the right side.   
      
   Repeat until there aren't any more to mark.   
      
   Any unmarked rules don't ever reach a terminal.   
   [It's not quite that, it's rules that have no nonterminals, that   
   is, either just terminals or empty.  This will recognize a   
   possibly empty sequence of x's:   
      
   A: /* nothing */   
   A: x A   
      
   -John]   
      
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