         let reducer = AnyValueReducer(
             fetch: { db -> Int in
                 fetchCount += 1
+                // test for database access
                 return try Int.fetchOne(db, "SELECT COUNT(*) FROM t")!
             },
             value: { count -> String? in
                 reduceCount += 1
                 if dropNext {
+                    // test that reducer can drop values
                     dropNext = false
                     return nil
                 }
-                // just make sure the fetched type and the notified type can be different
+                // test that the fetched type and the notified type can be different
                 return count.description
             })
         

                    return .commit
                }
                
                // A rollbacked transaction
                try db.inTransaction {
                    try db.execute("INSERT INTO t DEFAULT VALUES")
                    return .rollback
                }

                // A 2nd notified transaction
                try db.inTransaction {
                    try db.execute("INSERT INTO t DEFAULT VALUES")
                    try db.execute("INSERT INTO t DEFAULT VALUES")

            XCTAssertEqual(changes, ["0"])
            
            try dbQueue.inDatabase { db in
                // Test a 1st notified transaction
                try db.inTransaction {
                    try db.execute("INSERT INTO t DEFAULT VALUES")
                    return .commit
                }
                
                // Test an untracked transaction
                try db.inTransaction {
                    try db.execute("CREATE TABLE ignored(a)")
                    return .commit
                }
                
                // Test a dropped transaction
                dropNext = true
                try db.inTransaction {
                    try db.execute("INSERT INTO t DEFAULT VALUES")
