diff --git a/src/bin/flat/main.rs b/src/bin/flat/main.rs
index 04be948..a3e28bb 100644
--- a/src/bin/flat/main.rs
+++ b/src/bin/flat/main.rs
@@ -627,9 +627,6 @@ impl Tube {
 mod fns {
 	use crate::FloatExt2;
 
-	#[cfg(test)]
-	use approx::{abs_diff_eq, assert_abs_diff_eq};
-
 	pub struct TubeX {
 		pub min: f32,
 		pub max: f32,
@@ -667,44 +664,49 @@ mod fns {
 		pub fn d2u(&self, x: f32) -> f32 { extend_const(x, |x| -2.0 * x.signum() * self.a() * self.root(x).powi(-3), self.external, 0.0) }
 	}
 
-	#[test]
-	fn test_tube_y() {
-		let testee = TubeY { internal: 100.0, external: 150.0 };
-		let ε = 1.0e-4f32;
-		let δ = 1.0 / 8.0; // Mathematically, you want this to be small. Computationally, you don’t.
-		let margin = 1.0 / 16.0;
-		let mul = 1.0 + margin;
-		assert_abs_diff_eq!(testee.u(testee.external), testee.internal, epsilon = ε);
-		assert_abs_diff_eq!(testee.u(-testee.external), -testee.internal, epsilon = ε);
-		assert_abs_diff_eq!(testee.du(testee.external), 1., epsilon = ε);
-		assert_abs_diff_eq!(testee.du(-testee.external), 1., epsilon = ε);
-		for x in itertools_num::linspace(-mul * testee.external, mul * testee.external, 100) {
-			let ux = testee.u(x);
-			let xux = testee.x(ux);
-			assert!(abs_diff_eq!(x, xux, epsilon = ε), "At x={}:\nu(x):    {}\nx(u(x)): {}\n", x, ux, xux);
+	#[cfg(test)]
+	mod test {
+		use approx::{abs_diff_eq, assert_abs_diff_eq};
 
-			let du_num = (testee.u(x + δ) - testee.u(x - δ)) / (2. * δ);
-			let du_expl = testee.du(x);
-			assert!(abs_diff_eq!(du_expl, du_num, epsilon = ε), "At x={}, du/dx:\nnumerical: {}\nexplicit:  {}\n", x, du_num, du_expl);
+		#[test]
+		fn test_tube_y() {
+			let testee = super::TubeY { internal: 100.0, external: 150.0 };
+			let ε = 1.0e-4f32;
+			let δ = 1.0 / 8.0; // Mathematically, you want this to be small. Computationally, you don’t.
+			let margin = 1.0 / 16.0;
+			let mul = 1.0 + margin;
+			assert_abs_diff_eq!(testee.u(testee.external), testee.internal, epsilon = ε);
+			assert_abs_diff_eq!(testee.u(-testee.external), -testee.internal, epsilon = ε);
+			assert_abs_diff_eq!(testee.du(testee.external), 1., epsilon = ε);
+			assert_abs_diff_eq!(testee.du(-testee.external), 1., epsilon = ε);
+			for x in itertools_num::linspace(-mul * testee.external, mul * testee.external, 100) {
+				let ux = testee.u(x);
+				let xux = testee.x(ux);
+				assert!(abs_diff_eq!(x, xux, epsilon = ε), "At x={}:\nu(x):    {}\nx(u(x)): {}\n", x, ux, xux);
 
-			let dudx = du_expl * testee.dx(ux);
-			assert!(abs_diff_eq!(dudx, 1.0, epsilon = ε), "At x={}:\ndu/dx * dx/du: {}\n", x, dudx);
+				let du_num = (testee.u(x + δ) - testee.u(x - δ)) / (2. * δ);
+				let du_expl = testee.du(x);
+				assert!(abs_diff_eq!(du_expl, du_num, epsilon = ε), "At x={}, du/dx:\nnumerical: {}\nexplicit:  {}\n", x, du_num, du_expl);
 
-			let d2u_num = (testee.du(x + δ) - testee.du(x - δ)) / (2. * δ);
-			let d2u_expl = testee.d2u(x);
-			assert!(abs_diff_eq!(d2u_expl, d2u_num, epsilon = ε), "At x={}, d^2u/dx^2:\nnumerical: {}\nexplicit:  {}\n", x, d2u_num, d2u_expl);
-		}
-		for u in itertools_num::linspace(-mul * testee.internal, mul * testee.internal, 100) {
-			let xu = testee.x(u);
-			let uxu = testee.u(xu);
-			assert!(abs_diff_eq!(u, uxu, epsilon = ε), "At u={}:\nx(u):    {}\nu(x(u)): {}\n", u, xu, uxu);
+				let dudx = du_expl * testee.dx(ux);
+				assert!(abs_diff_eq!(dudx, 1.0, epsilon = ε), "At x={}:\ndu/dx * dx/du: {}\n", x, dudx);
 
-			let dx_num = (testee.x(u + δ) - testee.x(u - δ)) / (2. * δ);
-			let dx_expl = testee.dx(u);
-			assert!(abs_diff_eq!(dx_expl, dx_num, epsilon = ε), "At u={}, dx/du:\nnumerical: {}\nexplicit:  {}\n", u, dx_num, dx_expl);
+				let d2u_num = (testee.du(x + δ) - testee.du(x - δ)) / (2. * δ);
+				let d2u_expl = testee.d2u(x);
+				assert!(abs_diff_eq!(d2u_expl, d2u_num, epsilon = ε), "At x={}, d^2u/dx^2:\nnumerical: {}\nexplicit:  {}\n", x, d2u_num, d2u_expl);
+			}
+			for u in itertools_num::linspace(-mul * testee.internal, mul * testee.internal, 100) {
+				let xu = testee.x(u);
+				let uxu = testee.u(xu);
+				assert!(abs_diff_eq!(u, uxu, epsilon = ε), "At u={}:\nx(u):    {}\nu(x(u)): {}\n", u, xu, uxu);
 
-			let dudx = testee.du(xu) * dx_expl;
-			assert!(abs_diff_eq!(dudx, 1.0, epsilon = ε), "At u={}:\ndu/dx * dx/du: {}\n", u, dudx);
+				let dx_num = (testee.x(u + δ) - testee.x(u - δ)) / (2. * δ);
+				let dx_expl = testee.dx(u);
+				assert!(abs_diff_eq!(dx_expl, dx_num, epsilon = ε), "At u={}, dx/du:\nnumerical: {}\nexplicit:  {}\n", u, dx_num, dx_expl);
+
+				let dudx = testee.du(xu) * dx_expl;
+				assert!(abs_diff_eq!(dudx, 1.0, epsilon = ε), "At u={}:\ndu/dx * dx/du: {}\n", u, dudx);
+			}
 		}
 	}
 }