From: cdodd@acm.org   
      
   philip.k.chow@gmail.com wrote in news:08-11-055@comp.compilers:   
       (grammar deleted)   
   > Here is a tricky sentence that demonstrates why it's hard:   
   >   
   > all a: all b, c: all d | x   
   >   
   > This has only one legal parse (parentheses added for clarification):   
   >   
   > all (a:all b), (c:all d) | x   
      
   To understand precisely why this is so hard, consider what happens if you   
   add a |-suffix to the end:   
      
   all a: all b, c: all d | x | y   
      
   now the parse looks completely different:   
      
   all (a: all (b, c : all d) | x) | y   
      
   So the problem is that you need to be able to look ahead to the end of the   
   input to see how many |-suffixes there are in order to figure out how to   
   parse the beginning of the expression.  This makes it pretty much impossible   
   to parse without unbounded lookahead, or backtracking, or something else.   
      
   Chris Dodd   
   cdodd@acm.org   
      
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