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                          Exercise 2
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 1. Decidable languages are closed under complementation. (that 
is if L is decidable that the complement of L is decidable). Why isn't it
possible to show that recognizable languages are also closed under
complementation (i.e., what goes wrong if you try to prove this)?


 2. What about Set Union and Set Interaction? Are decidable languages closed
under them? What about Recognizable languages?