diff --git a/016-closures/closures.go b/016-closures/closures.go
index f034e34..a9766e6 100644
--- a/016-closures/closures.go
+++ b/016-closures/closures.go
@@ -18,3 +18,5 @@ func main() {
     fmt.Println(nextEven())
     fmt.Println(nextEven())
 }
+
+// todo: note example of first-class functions.
diff --git a/tool/index.txt b/tool/index.txt
index 70c6000..8283750 100644
--- a/tool/index.txt
+++ b/tool/index.txt
@@ -46,10 +46,13 @@ rate-limiting
 worker-pools
 ~ nonblocking-sends-and-receives
 ~ closing-channels
+timers
+tickers
 
 # core libraries
 sorting
 sorting-by-functions
+collection-functions
 string-functions
 string-formatting
 regexs
@@ -60,16 +63,13 @@ time
 ~ time-formatting
 epochs
 elapsed-time
-timers
-tickers
 random-numbers
 number-parsing
-collection-functions
 urls
 sha1-hashes
 ~ base64-encoding
 
-# systems
+# systems programming
 file-open
 file-read
 file-write
@@ -85,7 +85,7 @@ users
 line-filter
 tcp server
 
-# network clients
+# client libraries
 tcp-client
 http-client
 http-client-basic