State that Mailbox objects are iterator objects.
diff --git a/Doc/lib/libmailbox.tex b/Doc/lib/libmailbox.tex
index 9047a61..a142d48 100644
--- a/Doc/lib/libmailbox.tex
+++ b/Doc/lib/libmailbox.tex
@@ -86,8 +86,8 @@
\subsection{Mailbox Objects \label{mailbox-objects}}
-All implementations of Mailbox objects have one externally visible
-method:
+All implementations of Mailbox objects are iterator objects, and so
+have one externally visible method:
\begin{methoddesc}[mailbox]{next}{}
Return the next message in the mailbox, created with the optional