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 Solution for hw10. Type Derivations in Lecture Note 10

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For each problem, you have to write 
        * constraint system
        * decide if lambda-term has a simple type or not
          by doing unification of the constraints
        * give a type derivation and decide the type of
          lambda-term if it has a simple type.

This is a summary table:
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 NO     typable         type of lambda-term
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  1     Yes             (r->(s->t))->((r->s)->(r->t))
  2     Yes             (r->r)->(r->r)
  3     No
  4     No
  5     No
  6     No
  7     Yes             (r->r)->(r->r)
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