diff --git a/README.md b/README.md
index 36d79eb..2f20fb4 100644
--- a/README.md
+++ b/README.md
@@ -19,6 +19,7 @@ mongosh --port 27017
 > db.users.find({age: {$gt: 25}}).toArray()
 > db.users.updateOne({name: "alice"}, {$set: {vip: true}})
 > db.users.deleteOne({name: "bob"})
+> db.sessions.createIndex({expireAt: 1}, {expireAfterSeconds: 3600})
 ```
 
 ## Features
@@ -41,8 +42,9 @@ mongosh --port 27017
 - **Secondary indexes**: `createIndex`/`listIndexes`/`dropIndex` via the
   three driver commands, single-field and compound, with `unique`,
   `sparse` and `expireAfterSeconds` (TTL) options, persisted in the log
-  and rebuilt on open (compaction
-  re-emits them). The query planner turns equality / `$in` / range
+  and rebuilt on open (compaction re-emits them). A background sweeper
+  expires TTL-indexed documents through the ordinary logged write path.
+  The query planner turns equality / `$in` / range
   predicates into index lookups across `find`, `count`, `update`,
   `delete`, `findAndModify`, and a leading `$match` in `aggregate`; every
   candidate is re-checked against the full filter, so an index that
@@ -101,9 +103,10 @@ the query planner to narrow scans.
   `expireAfterSeconds` must be a whole number in `[0, 2147483647]` (`0`
   means "expire at the stored instant"), a non-date value at the path never
   expires, an array of dates expires on its earliest member, and expiry is
-  coarse: a document stays visible until the next sweep. Changing the
-  expiry of an existing index is `IndexOptionsConflict` — `collMod` is not
-  implemented.
+  coarse: a document stays visible until the next sweep. Re-creating an
+  index with a different expiry is `IndexOptionsConflict` (85) and an
+  expiry on `{_id: 1}` is `InvalidIndexSpecificationOption` (197), both as
+  MongoDB has them.
 - **Multikey**: an array at an indexed path is indexed as a whole *and*
   element-wise, mirroring the query matcher exactly, so both
   `{tags: "a"}` and `{tags: ["a","b"]}` hit the index. A compound index
@@ -121,7 +124,11 @@ the query planner to narrow scans.
 
 v1 limits: no index-accelerated sort, no hashed/text/geo/partial indexes,
 and entry insert/removal is O(n) (a sorted array) — fine for a light
-database, with a B-tree or id→entry map as the follow-up.
+database, with a B-tree or id→entry map as the follow-up. A TTL sweep
+walks every entry of every TTL index and holds the write lock for the
+whole pass, so the interval is the tuning knob: expiry is never more
+precise than `--ttl-sweep-secs`, and a very large TTL index wants a
+longer one.
 
 ## Not (yet) implemented
 
@@ -129,6 +136,8 @@ database, with a B-tree or id→entry map as the follow-up.
 - Real cursors (all results are returned in one batch, cursor id 0)
 - Transactions, change streams, replicasets
 - Compression (OP_COMPRESSED)
+- `collMod`, so an index's `expireAfterSeconds` cannot be changed in
+  place — drop the index and re-create it with the new expiry
 - `dropCollection`/`dropDatabase` write no log record, so a dropped
   collection (and its index definitions) resurrect on restart; and
   compaction never resets `log_bytes`, so every write after the first