lib/PublicInbox/Msgmap.pm | 1 +

diff --git a/lib/PublicInbox/Msgmap.pm b/lib/PublicInbox/Msgmap.pm
index d696ce837395bcbc0800a09e41290f8f1e5fee02..f15875e37cd8a88c2b74956301892d4198be541f 100644
--- a/lib/PublicInbox/Msgmap.pm
+++ b/lib/PublicInbox/Msgmap.pm
@@ -91,6 +91,7 @@ 	my ($self, $version, $i, $commit) = @_;
 	$self->meta_accessor("last_xap$version-$i", $commit);
 }
 
+# this is the UIDVALIDITY for IMAP (cf. RFC 3501 sec 2.3.1.1. item 3)
 sub created_at {
 	my ($self, $second) = @_;
 	$self->meta_accessor('created_at', $second);