- * Currenly one version of this:
- * - vcache - catchall implementation, decomposes to TRI/LINE/POINT prims
- * Later:
- * - varray, varray_split
- * - velement, velement_split
+ * The fetch elements are indices to the vertices. The draw elements are
+ * indices to the fetched vertices. When both arrays of elements are both
+ * linear, middle->run_linear is called; When only the fetch elements are
+ * linear, middle->run_linear_elts is called; Otherwise, middle->run is
+ * called.
+ *
+ * When the number of the draw elements exceeds max_vertex of the middle end,
+ * the draw elements (as well as the fetch elements) are splitted and the
+ * middle end is called multiple times.