examples: add tree_of_nodes.v, based on a type Tree = Leaf | Node sumtype

examples/tree_of_nodes.v

@@ -0,0 +1,27 @@
+type Tree = Leaf | Node
+
+struct Leaf {}
+
+struct Node {
+	value int
+	left  Tree
+	right Tree
+}
+
+// NB: a match expression, infers the type of its result
+// from the type of the return value in the first branch,
+// => it needs an explicit int(0) cast here:
+fn size(tree Tree) int {
+	return match tree {
+		Leaf { int(0) }
+		Node { 1 + size(tree.left) + size(tree.right) }
+	}
+}
+
+fn main() {
+	node1 := Node{30, Leaf{}, Leaf{}}
+	node2 := Node{20, Leaf{}, Leaf{}}
+	tree := Node{10, node1, node2}
+	println('tree structure:\n $tree')
+	println('tree size: ${size(tree)}')
+}