,
if index>=0, index is counted from left to right. and the index of leftest element is 0, the rightest's is z-1. z is the size of the list
if index<0, index is counted from right to left, and the value of rightest element is -1, the leftest's is z.
ex: the value of li[0]=li[-4]="a",li[4]=li-1|="example";