diff --git a/Doc/tools/toc2bkm.py b/Doc/tools/toc2bkm.py
index ed78f36c37f..0a17f46d13b 100755
--- a/Doc/tools/toc2bkm.py
+++ b/Doc/tools/toc2bkm.py
@@ -16,6 +16,8 @@
 # Ench item in an entry is a tuple of:
 #
 #   Section #,  Title String,  Page #,  List of Sub-entries
+#
+# The return value of parse_toc() is such a tuple.
 
 cline_re = r"""^
 \\contentsline\ \{([a-z]*)}             # type of section in $1