Unique because that particular permutation has not appeared before. 之所以独一无二,是因为那种独特的基因排列组合以前不曾出现过。
Besides, the author understood the rule of negative multiplication completely and studied the problem of permutation and combination deeply. 另外,萨玛瓦尔完全理解了负数的乘法法则,并对排列组合问题也有充分的认识。