[add new example pow2
Helmut Grohne <grohne@cs.uni-bonn.de>**20150310142123
 Ignore-this: ceb301bd964f7941ceb6b325403df57d
 
 It computes powers of two. This seems to be unsolvable to Igor at this point.
 Introducing an accumulator and replacing all Z with the accumulator lets Igor
 solve this quickly.
] hunk ./expl/Examples.hs 1747
+
+pow2 :: Nat -> Nat
+pow2 Z = S Z
+pow2 (S Z) = S(S Z)
+pow2 (S(S Z)) = S(S(S(S Z)))
+pow2 (S(S(S Z))) = S(S(S(S(S(S(S(S Z)))))))
+--pow2 (S(S(S(S Z)))) = S(S(S(S(S(S(S(S(S(S(S(S(S(S(S(S Z)))))))))))))))