This is the mnemonic method I use. It is just an approach of what is happening, but it works.
Don't think of those as indexes. Think of them as offsets on a circular list.
Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:
You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).
If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)
