+The callback is expected to return 0 if everything went well, which is the
+same semantic than the "main()" function. You can also build stacks of
+callbacks. It is perfectly valid to register several callbacks to a given
+message. When a message arrives, it is passed to the firstly-registered
+callback. If the callback returns 0, it means that it consumed the message,
+which is discarded. If the callback returns 1 (or any other "true" value),
+the message is passed to the next callback of the stack, and so on. At the
+end, if no callback returned 0, an exception is raised.
+
+This mecanism can for example be used to introduce dupplication and replay.
+You add a callback simply in charge of storing the message in a database,
+and since it returns 1, the message is then passed to the "real" callback.
+To be perfectly honest, I never had use of this functionnality myself, but I
+feel it could be useful in some cases...
+
+\subsection GRAS_tut_tour_message_recaping_rpc_aside2 Aside: Exceptions and callbacks
+
+One of the parts I'm the most proud of in GRAS is this one: imagine you have
+a rpc callback executing on a remote server. If this callback raises an
+exception, it will be propagated on the network back to the client, and
+revived there. So, the client will get automatically any exception raised by
+the server. Cool, isn't it? Afterward, simply refer to the <tt>host</tt>
+field of the #xbt_ex_t to retrieve the machine on which it was initialy
+raised.
+
+In case you wonder about the exceptions I'm speaking about (after all,
+SimGrid is in C ANSI, and there is usually no exception mecanism in C ANSI),
+you may want to refer to the section \ref XBT_ex. Note that exception can be
+troublesome to use (mainly because the compiler won't catch your mistakes on
+this).
+
+\subsection GRAS_tut_tour_message_recaping_rpc_aside3 Aside: Symbol versionning (TODO)
+
+This section covers a point not explicited elsewhere in the documentation.
+It may be seen as a bit hardcore, and you should probably skip it the first
+time you read the tutorial.
+
+\section GRAS_tut_tour_message_recaping_sync Syncronization messages in GRAS (TODO)