diff -r 1f1b946fc656 -r 794833f18a05 src/mapping/dataterm.rs
--- a/src/mapping/dataterm.rs	Fri Nov 28 13:28:17 2025 -0500
+++ b/src/mapping/dataterm.rs	Fri Nov 28 14:01:14 2025 -0500
@@ -64,6 +64,16 @@
     pub fn fidelity(&self) -> &'_ G {
         &self.g
     }
+
+    /// Returns the residual $Ax-b$.
+    pub fn residual<'a, 'b>(&'b self, x: &'a Domain) -> <A as Mapping<Domain>>::Codomain
+    where
+        &'a Domain: Instance<Domain>,
+        <A as Mapping<Domain>>::Codomain:
+            Sub<&'b <A as Mapping<Domain>>::Codomain, Output = <A as Mapping<Domain>>::Codomain>,
+    {
+        self.opA.apply(x) - &self.b
+    }
 }
 
 //+ AdjointProductBoundedBy<RNDM<N, F>, P, FloatType = F>,