+/** \addtogroup GRAS_globals
+ * \brief Handling global variables so that it works on simulator.
+ *
+ * In GRAS, using globals is forbidden since the "processes" will
+ * sometimes run as a thread inside the same process (namely, in
+ * simulation mode). So, you have to put all globals in a structure, and
+ * let GRAS handle it.
+ *
+ * Use the \ref gras_userdata_new macro to create a new user data (or malloc it
+ * and use \ref gras_userdata_set yourself), and \ref gras_userdata_get to
+ * retrieve a reference to it.
+ *