Adds example for non-equivalence of take operations

examples/equivalent_operations/Take.curry

+--- Example to show the non-equivalence of two versions of the
+--- `take` operation.
+
+import Test.Prop
+
+-- take operation from the Prelude:
+take1 :: Int -> [a] -> [a]
+take1 n xs | n <= 0    = []
+           | otherwise = takep n xs
+   where takep _ []     = []
+         takep m (y:ys) = y : take1 (m-1) ys
+
+-- take operation with different pattern matching:
+take2 :: Int -> [a] -> [a]
+take2 n [] = []
+take2 n (x:xs) | n > 0     = x : take2 (n-1) xs
+               | otherwise = []
+
+-- The equality of both take operations on ground values always succeeds:
+take_groundequiv x xs = take1 x xs <~> take2 x xs
+
+-- Actually, take1 and take2 are not equivalent, as can be seen
+-- by evaluating `take... failed [])` or `take... 0 failed`.
+-- Thus, we check the equivalence with CurryCheck:
+
+-- In PAKCS, the counter example is reported by the 11th test:
+take_equiv = take1 <=> take2