diff --git a/parse.y b/parse.y
index 383b5ef03b..b6df72fa55 100644
--- a/parse.y
+++ b/parse.y
@@ -6132,6 +6132,8 @@ heredoc_dedent(struct parser_params *p, VALUE array)
  *  call-seq:
  *    Ripper.dedent_string(input, width)   -> Integer
  *
+ *  USE OF RIPPER LIBRARY ONLY.
+ *
  *  Strips up to +width+ leading whitespaces from +input+,
  *  and returns the stripped column width.
  */