diff --git a/src/bin/flat/tube/mod.rs b/src/bin/flat/tube/mod.rs
index 6b256c0..846a7a0 100644
--- a/src/bin/flat/tube/mod.rs
+++ b/src/bin/flat/tube/mod.rs
@@ -171,22 +171,35 @@ impl Space {
 	}
 }
 
+/// Like [`std::iter::successors`] but with an upper limit on iteration count.
+///
+/// # Panics
+///
+/// Panics if the sequence doesn’t terminate in `max_iters` calls of `succ`.
+fn iterate_with_limit<T>(max_iters: usize, init: T, mut succ: impl FnMut(T) -> Option<T>) {
+	let mut state = init;
+	for _ in 0..max_iters {
+		match succ(state) {
+			Some(next) => state = next,
+			None => return,
+		}
+	}
+	panic!("iteration limit exceeded");
+}
+
 impl Traceable for Space {
 	fn trace(&self, camera: Location, ray: Ray) -> Vec<Hit> {
 		let ray = self.camera_ray_to_abs(camera, ray);
 		let mut hits = vec![];
-		std::iter::successors(Some(ray), |ray: &Ray| {
+		iterate_with_limit(100, ray, |ray| {
 			let ret = self
-				.trace_iter(*ray)
+				.trace_iter(ray)
 				.skip(1)
 				.find_map(|ray| self.obj_hitter(ray.pos).map(|hitter| hitter(self, ray)))
 				.expect("Space::trace_iter does not terminate");
 			hits.extend(ret.objects); // TODO fix distance
 			ret.end
-		})
-		.nth(100)
-		.is_some()
-		.then(|| panic!("tracing didn't terminate"));
+		});
 		hits
 	}
 }