diff --git a/src/bin/flat/main.rs b/src/bin/flat/main.rs
index 2ec8e8f..0073387 100644
--- a/src/bin/flat/main.rs
+++ b/src/bin/flat/main.rs
@@ -684,6 +684,8 @@ mod fns {
 			let du = (testee.u(x + δ) - testee.u(x - δ)) / (2. * δ);
 			assert!(abs_diff_eq!(du, testee.du(x), epsilon = ε), "At x={}, u':\nexpected: {}\nactual:   {}\n", x, du, testee.du(x));
 
+			assert!(abs_diff_eq!(testee.du(x) * testee.dx(u), 1.0, epsilon = ε), "At x={}:\nu'x': {}\n", x, testee.du(x) * testee.dx(u));
+
 			let d2u = (testee.du(x + δ) - testee.du(x - δ)) / (2. * δ);
 			assert!(abs_diff_eq!(d2u, testee.d2u(x), epsilon = ε), "At x={}, u'':\nexpected: {}\nactual:   {}\n", x, d2u, testee.d2u(x));
 		}
@@ -693,6 +695,8 @@ mod fns {
 
 			let dx = (testee.x(u + δ) - testee.x(u - δ)) / (2. * δ);
 			assert!(abs_diff_eq!(dx, testee.dx(u), epsilon = ε), "At u={}, x':\nexpected: {}\nactual:   {}\n", u, dx, testee.dx(u));
+
+			assert!(abs_diff_eq!(testee.du(x) * testee.dx(u), 1.0, epsilon = ε), "At u={}:\nu'x': {}\n", u, testee.du(x) * testee.dx(u));
 		}
 	}
 }