Example added

examples/equivalent_operations/Unzip.curry

+--- Example from:
+--- 
+--- Olaf Chitil:
+--- StrictCheck: a Tool for Testing Whether a Function is Unnecessarily Strict
+--- University of Kent, TR 2-11, 2011
+--- https://kar.kent.ac.uk/30756/
+
+import Test.Prop
+
+--- Definition of unzip from the standard prelude.
+unzip1 :: [(a,b)] -> ([a],[b])
+unzip1 []         = ([],[])
+unzip1 ((x,y):ps) = (x:xs,y:ys) where (xs,ys) = unzip1 ps
+
+--- Definition from Chitil'11:
+unzip2 :: [(a,b)] -> ([a],[b])
+unzip2 xs = foldr (\(a,b) (as,bs) -> (a:as,b:bs)) ([],[]) xs
+
+-- These operations are not equivalent:
+unzipEquiv = unzip1 <=> unzip2