+/** \brief Add the value delta to var->df (the sum of latencies).
+ *
+ * \param sys the lmm_system_t associated
+ * \param var the lmm_variable_t which need to updated
+ * \param delta the variation of the latency
+ *
+ * Add the value delta to var->df (the sum of latencys associated to the
+ * flow). Whenever this function is called a change is signed in the system. To
+ * avoid false system changing detection it is a good idea to test
+ * (delta != 0) before calling it.
+ *
+ */