examples: use Empty instead of Leaf in tree_of_nodes.v

examples/tree_of_nodes.v

@@ -1,6 +1,6 @@
-type Tree = Leaf | Node
+type Tree = Empty | Node
 
-struct Leaf {}
+struct Empty {}
 
 struct Node {
 	value int
@@ -13,14 +13,14 @@ struct Node {
 // => it needs an explicit int(0) cast here:
 fn size(tree Tree) int {
 	return match tree {
-		Leaf { int(0) }
+		Empty { int(0) }
 		Node { 1 + size(tree.left) + size(tree.right) }
 	}
 }
 
 fn main() {
-	node1 := Node{30, Leaf{}, Leaf{}}
-	node2 := Node{20, Leaf{}, Leaf{}}
+	node1 := Node{30, Empty{}, Empty{}}
+	node2 := Node{20, Empty{}, Empty{}}
 	tree := Node{10, node1, node2}
 	println('tree structure:\n $tree')
 	println('tree size: ${size(tree)}')