Merge branch 'exo'

Exercices on recursive types.
Exercices with list and binary tree.

list.hs

@@ -0,0 +1,6 @@
+data List = Ground | Leaf Integer List
+
+hyperSum xs = case xs of
+	Ground -> 0
+	Leaf x tail -> x + hyperSum tail
+

tree.hs

@@ -0,0 +1,6 @@
+data BinaryTree = Leaf | Node Integer BinaryTree BinaryTree
+
+height bigTree = case bigTree of
+	Leaf -> 0
+	Node _ leftTree rightTree -> 1 + max (height leftTree) (height rightTree)
+