From: gene.ressler@gmail.com   
      
   > No, context-free grammars are not closed under complement, so your   
   > claim is only true for special cases in which the complement of a   
   > context-sensitive grammar might be context-free.   
      
   I'm sorry you didn't understand what I wrote.  I only said the   
   complement of a^n b^n a^n is context free (even though the set itself   
   isn't by the pumping lemma for CFLs).  I described how to build a CFG   
   for that complement.  This grammar is a proof of CF-ness.   
      
   A small additional point is that you no doubt meant CFL's (rather than   
   CFGs) are not closed under complement.   Complement is defined on   
   sets, and a CFG is not a set.   
      
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