improve the ntl drawing
* the list can be allocated as
* p = ntl_calloc(n, size);
*
- *
- * +-----------------+
- * | |
+ * /-indices[n+1]\ /--------- e[n]-----------\
* p -> [ | | | | | | |0][e_0]...............[e_(n-1)]
- * \--indices[n+1]-/ \--------- e[n]-----------/
+ * | ^
+ * | |
+ * +---------->------+
*
* p points to the begin of the memory block which overlaps with indices.
*
