diff --git a/src/bin/flat/main.rs b/src/bin/flat/main.rs
index ec89f8c..04be948 100644
--- a/src/bin/flat/main.rs
+++ b/src/bin/flat/main.rs
@@ -628,7 +628,7 @@ mod fns {
 	use crate::FloatExt2;
 
 	#[cfg(test)]
-	use approx::abs_diff_eq;
+	use approx::{abs_diff_eq, assert_abs_diff_eq};
 
 	pub struct TubeX {
 		pub min: f32,
@@ -674,6 +674,10 @@ mod fns {
 		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);