diff --git a/src/run.rs b/src/run.rs
index 7412191..af0e4cc 100644
--- a/src/run.rs
+++ b/src/run.rs
@@ -1,3 +1,4 @@
+// step-3 measurement probe: comment-only change, no semantic content
 use std::cell::RefCell;
 use std::collections::{BTreeMap, BTreeSet, VecDeque};
 use std::ffi::{OsStr, OsString};