add first exercise of lecture 3

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diff --git a/lec03/Golf.hs b/lec03/Golf.hs
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+module Golf where
+
+-- Exercise 1
+
+-- Explanation:
+-- takeEvery takes all elements with index equal or multiple of a given
+-- positive number, index 0 exluded. In the first two cases, it checks whether
+-- the list is empty or the number is zero, and returns an empty list if so. In
+-- the next case, which is the most general, it defines l2 and l3, which are
+-- respectively the list without the first n-1 and n elements. Then it takes
+-- the fist element out of l2, which, relative to the list, has an index
+-- multiple of (or equal to) the given number, and then recursively call the
+-- function itself on l3 so that it drops another (n-1) elements and takes the
+-- one whose index is multiple of the number, etc.
+takeEvery :: [a] -> Int -> [a]
+takeEvery [] _ = []
+takeEvery _ 0 = []
+takeEvery l n = take 1 l2 ++ takeEvery l3 n
+    where
+        l2 = drop (n-1) l
+        l3 = drop 1 l2
+
+-- Explanation:
+-- skips calls takeEvery for every number from 1 to the length of the list - 1.
+skips :: [a] -> [[a]]
+skips [] = []
+skips l = map (takeEvery l) [1..length l]