+/** Perform automatic term rewriting rules in this class. In the following
+ * x, x1, x2,... stand for a symbolic variables of type ex and c, c1, c2...
+ * stand for such expressions that contain a plain number.
+ * - *(...,x;0) -> 0
+ * - *(+(x1,x2,...);c) -> *(+(*(x1,c),*(x2,c),...))
+ * - *(x;1) -> x
+ * - *(;c) -> c
+ *
+ * @param level cut-off in recursive evaluation */