Use Box<T> to break cycle

src/main.rs

@@ -1,12 +1,10 @@
 use hello::geometry::Geometry;
-use hello::math::{Mat4x4, Point3};
+use hello::math::Mat4x4;
 
-/*
 enum SceneGraphNode {
     Geometry(Mat4x4, Geometry),
-    SubNode(Mat4x4, SceneGraphNode),
+    SubNode(Mat4x4, Box<SceneGraphNode>),
 }
-*/
 
 fn main() {
     let v1 = 5;