diff --git a/libguile/guile-snarf.in b/libguile/guile-snarf.in
index 333c40edc..942591465 100644
--- a/libguile/guile-snarf.in
+++ b/libguile/guile-snarf.in
@@ -33,6 +33,9 @@
 # If there are errors during processing, delete OUTFILE and exit with
 # non-zero status.
 #
+# During snarfing, the pre-processor macro @code{SCM_MAGIC_SNARFER} is
+# defined.
+#
 # If env var CPP is set, use its value instead of the C pre-processor
 # determined at Guile configure-time: "@CPP@".