diff --git a/src/bin/flat.rs b/src/bin/flat.rs
index a13bba0..8a6b76a 100644
--- a/src/bin/flat.rs
+++ b/src/bin/flat.rs
@@ -250,6 +250,12 @@ fn test_rect() {
 	assert_abs_diff_eq!(r.x(-r.internal_halflength), -r.external_halflength, epsilon = 1.0e-5);
 	assert_abs_diff_eq!(r.u(r.external_halflength), r.internal_halflength, epsilon = 1.0e-5);
 	assert_abs_diff_eq!(r.u(-r.external_halflength), -r.internal_halflength, epsilon = 1.0e-5);
+
+	assert_abs_diff_eq!(r.dx(r.internal_halflength, 3.0), 3.0, epsilon = 1.0e-5);
+	assert_abs_diff_eq!(r.dx(-r.internal_halflength, 3.0), 3.0, epsilon = 1.0e-5);
+	assert_abs_diff_eq!(r.du(r.external_halflength, 3.0), 3.0, epsilon = 1.0e-5);
+	assert_abs_diff_eq!(r.du(-r.external_halflength, 3.0), 3.0, epsilon = 1.0e-5);
+
 	assert_abs_diff_eq!(r.u(r.x(1.0)), 1.0, epsilon = 1.0e-5);
 	assert_abs_diff_eq!(r.u(r.x(5.0)), 5.0, epsilon = 1.0e-5);
 	assert_abs_diff_eq!(r.u(r.x(-5.0)), -5.0, epsilon = 1.0e-5);