diff --git a/crates/tor-basic-utils/src/n_key_set.rs b/crates/tor-basic-utils/src/n_key_set.rs
index f72cf1f9e..f50aafe89 100644
--- a/crates/tor-basic-utils/src/n_key_set.rs
+++ b/crates/tor-basic-utils/src/n_key_set.rs
@@ -218,6 +218,32 @@ This could be more efficient in space and time.
         {
             self.[<$key _map>].get($key).copied().map(|old_idx| self.remove_at(old_idx).expect("inconsistent state"))
         }
+
+
+        #[doc = concat!("Modify the element with the given value for `", stringify!($key), " by applying `func` to it.")]
+        ///
+        /// `func` is allowed to change the keys for this value.  All indices
+        /// are updated to refer to the new keys.  If the new keys conflict with
+        /// any previous values, those values are replaced and returned in a
+        /// vector.
+        ///
+        /// If `func` causes the value to have no keys at all, then the value
+        /// itself is also removed and returned in the result vector.
+        ///
+        /// Note that because this function needs to copy all key values and check whether
+        /// they have changed, it is not terribly efficient.
+        $vis fn [<modify_by_$key>] <BorrowAsKey_, F_>(&mut self, $key: &BorrowAsKey_, func: F_) -> Vec<$V>
+        where
+            $KEY : std::borrow::Borrow<BorrowAsKey_>,
+            BorrowAsKey_: std::hash::Hash + Eq + ?Sized,
+            F_: FnOnce(&mut $V)
+        {
+            if let Some(idx) = self.[<$key _map>].get($key) {
+                self.modify_at(*idx, func)
+            } else {
+                Vec::new()
+            }
+        }
         )+
 
         /// Return an iterator over the elements in this container.
@@ -324,7 +350,7 @@ This could be more efficient in space and time.
                 $(
                     if let Some($key) = $crate::n_key_set!( @access(removed, ($($($flag)+)?) $key : $KEY $({$($source)+})?) ) {
                         let old_idx = self.[<$key _map>].remove($key);
-                        debug_assert_eq!(old_idx, Some(idx));
+                        assert_eq!(old_idx, Some(idx));
                     }
                 )*
                 Some(removed)
@@ -333,6 +359,72 @@ This could be more efficient in space and time.
             }
         }
 
+        /// Change the value at `idx` by applying `func` to it.
+        ///
+        /// `func` is allowed to change the keys for this value.  All indices
+        /// are updated to refer to the new keys.  If the new keys conflict with
+        /// any previous values, those values are replaced and returned in a
+        /// vector.
+        ///
+        /// If `func` causes the value to have no keys at all, then the value
+        /// itself is also removed and returned in the result vector.
+        ///
+        /// # Panics
+        ///
+        /// Panics if `idx` is not present in this set.
+        fn modify_at<F_>(&mut self, idx: usize, func: F_) -> Vec<$V>
+        where
+            F_: FnOnce(&mut $V)
+        {
+            let value = self.values.get_mut(idx).expect("invalid index");
+            $(
+            let [<orig_$key>] = $crate::n_key_set!( @access(value, ($($($flag)+)?) $key : $KEY $({$($source)+})?) )
+                .map(|elt| elt.to_owned()) ;
+            )+
+
+            func(value);
+
+            // Check whether any keys have changed, and whether there still are
+            // any keys.
+            $(
+                let [<new_$key>] = $crate::n_key_set!( @access( value, ($($($flag)+)?) $key : $KEY $({$($source)+})?) ) ;
+            )+
+            let keys_changed = $( [<orig_$key>].as_ref().map(std::borrow::Borrow::borrow) != [<new_$key>] )||+ ;
+
+            if keys_changed {
+                let found_any_keys = $( [<new_$key>].is_some() )||+ ;
+
+                // Remove this value from every place that it was before.
+                //
+                // We can't use remove_at, since we have changed the keys in the
+                // value: we have to remove them manually from each index
+                // instead.
+                $(
+                    if let Some(orig) = [<orig_ $key>] {
+                        let removed = self.[<$key _map>].remove(&orig);
+                        assert_eq!(removed, Some(idx));
+                    }
+                )+
+                // Remove the value from its previous place in the index.  (This
+                // results in an extra copy when we call insert(), but if we
+                // didn't do it, we'd need to reimplement `insert()`.)
+                let removed = self.values.remove(idx);
+                if found_any_keys {
+                    // This item belongs: put it back and return the vector of
+                    // whatever was replaced.j
+                    self.insert(removed)
+                } else {
+                    // This item does not belong any longer, since all its keys
+                    // were removed.
+                    vec![removed]
+                }
+            } else {
+                // We did not change any keys, so we know we have not replaced
+                // any items.
+                vec![]
+            }
+        }
+
         /// Re-index all the values in this map, so that the map can use a more
         /// compact representation.
         ///
@@ -550,6 +642,31 @@ mod test {
         set.check_invariants();
     }
 
+    #[test]
+    fn modify_value() {
+        let mut set: Tuple2Set<i32, i32> = (1..=100).map(|idx| (idx, idx * idx)).collect();
+        set.check_invariants();
+
+        let v = set.modify_by_first(&30, |elt| elt.1 = 256);
+        set.check_invariants();
+        // one element was replaced.
+        assert_eq!(v.len(), 1);
+        assert_eq!(v[0], (16, 256));
+        assert_eq!(set.by_second(&256).unwrap(), &(30, 256));
+        assert_eq!(set.by_first(&30).unwrap(), &(30, 256));
+
+        let v = set.modify_by_first(&30, |elt| *elt = (-100, -100));
+        set.check_invariants();
+        // no elements were replaced.
+        assert_eq!(v.len(), 0);
+        assert_eq!(set.by_first(&30), None);
+        assert_eq!(set.by_second(&256), None);
+        assert_eq!(set.by_first(&-100).unwrap(), &(-100, -100));
+        assert_eq!(set.by_second(&-100).unwrap(), &(-100, -100));
+
+        set.check_invariants();
+    }
+
     #[allow(dead_code)]
     struct Weekday {
         dow: u8,