diff --git a/flow/flow_doc.md b/flow/flow_doc.md
index 6c301b3..2dfb03b 100644
--- a/flow/flow_doc.md
+++ b/flow/flow_doc.md
@@ -19,7 +19,7 @@ and audio processing networks and the application of network state data.
 ```
 ```
 
-
+Use `$` prefix to a filename. Use proc_expand_filename() to prepend `proj_dir` to a file prefixed with `$`.
 
 ### Polyphonic Network.