+ * Each constraint contains several partially overlapping logical sets of elements:
+ * \li Disabled elements which variable's weight is zero. This variables are not at all processed by LMM, but eventually the corresponding action will enable it (at least this is the idea).
+ * \li Enabled elements which variable's weight is non-zero. They are utilized in some LMM functions.
+ * \li Active elements which variable's weight is non-zero (i.e. it is enabled) AND its element value is non-zero. LMM_solve iterates over active elements during resolution, dynamically making them active or unactive.
+ *