diff --git a/src/bin/flat.rs b/src/bin/flat.rs
index 1d70cf3..0659f7f 100644
--- a/src/bin/flat.rs
+++ b/src/bin/flat.rs
@@ -96,6 +96,10 @@ impl Space {
 		Location { pos: p, rot: corr * loc.rot }
 	}
 
+	fn trace_iter(&self, ray: Ray) -> impl Iterator<Item=Ray> + '_ {
+		std::iter::successors(Some(ray), |&ray| Some(self.trace_step(ray)))
+	}
+
 	fn trace_inner(&self, ray: Ray) -> Ray {
 		assert_eq!(self.which_subspace(ray.pos), Inner);
 		let cell = RectInside { rect: self.rect };
@@ -116,7 +120,7 @@ impl Space {
 
 	fn trace_boundary(&self, ray: Ray) -> Ray {
 		assert_eq!(self.which_subspace(ray.pos), Boundary);
-		std::iter::successors(Some(ray), |&ray| Some(self.trace_step(ray)))
+		self.trace_iter(ray)
 			.find(|&ray| self.which_subspace(ray.pos) != Boundary)
 			.expect("Can't get outta the wall!")
 	}