diff --git a/tests/charon_llbc/src/lib.rs b/tests/charon_llbc/src/lib.rs
index eb40e9f..d030903 100644
--- a/tests/charon_llbc/src/lib.rs
+++ b/tests/charon_llbc/src/lib.rs
@@ -1,11 +1,14 @@
 //! Minimal Rust crate used to smoke-test the charon_llbc Bazel rule.
 //! Kept deliberately small: `charon rustc --preset=aeneas` should translate
 //! this without hitting any unsupported features.
+//!
+//! MUTANT #3 (mutation-testing spike, rules_lean#23): `identity` adds a
+//! constant. Expect: KILLED — identity.spec requires y = x exactly.
 
 pub fn add(x: u32, y: u32) -> u32 {
     x + y
 }
 
 pub fn identity(x: u64) -> u64 {
-    x
+    x + 1
 }