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diff --git a/docs/history/15-o3/code/examples/tree.zc b/docs/history/15-o3/code/examples/tree.zc
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+++ b/docs/history/15-o3/code/examples/tree.zc
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+// a binary search tree where every node owns its children. there's no free
+// anywhere: when the root's block ends, the whole tree goes with it.
+struct node { int v; *node left, right; };
+
+*node.u leaf(int v) { *node n = new(node); n.v = v; return u*n; }
+// n is borrowed; storing a new leaf into it makes n the leaf's owner
+fn insert(*node.m n, int v)
+{
+	if (v < n.v) { if (n.left) insert(n.left, v); else n.left = leaf(v); }
+	else { if (n.right) insert(n.right, v); else n.right = leaf(v); }
+	return 0;
+}
+fn walk(*node n) { if (!n) return 0; walk(n.left); putn(n.v); putc(' '); walk(n.right); return 0; }
+int height(*node n) { if (!n) return 0; int l = height(n.left), r = height(n.right); return 1 + (l > r ? l : r); }
+
+fn main()
+{
+	let before = live_objects();
+	{
+		*node root = leaf(50);
+		int x = 7;
+		for (int i = 0; i < 30; i++) { x = (x * 37 + 11) % 100; insert(root, x); }
+		walk(root); putc(10);
+		puts("height "); putn(height(root)); puts(", live objects "); putn(live_objects() - before); putc(10);
+	}
+	puts("after the block: "); putn(live_objects() - before); putc(10);
+	return 0;
+}