How is this list comprehension evaluated?

[Arturo B]

Hello, I'm making Python mini-projects and now I'm making a Latin Square

(Latin Square: http://en.wikipedia.org/wiki/Latin_square)

So, I started watching example code and I found this question on Stackoverflow: 

http://stackoverflow.com/questions/5313900/generating-cyclic-permutations-reduced-latin-squares-in-python

It uses a list comprenhension to generate the Latin Square, I'm am a newbie to Python, and  I've tried to figure out how this is evaluated:

    a = [1, 2, 3, 4]
    n = len(a)
    [[a[i - j] for i in range(n)] for j in range(n)]

I don't understand how the "i" and the "j" changes.
On my way of thought it is evaluated like this:

    [[a[0 - 0] for 0 in range(4)] for 0 in range(4)]
    [[a[1 - 1] for 1 in range(4)] for 1 in range(4)]
    [[a[2 - 2] for 2 in range(4)] for 2 in range(4)]
    [[a[3 - 3] for 3 in range(4)] for 3 in range(4)]

But I think I'm wrong... So, could you explain me as above? It would help me a lot.

Thanks for reading!