How to select even or odd elements based on class name

Question

if you create html layout like so

Answer 1

The problem is that :odd and :even (and their CSS cousins :nth-child(odd) and :nth-child(even)) refer to the order in which the elements appear as children of their parent, not as children with that particular selector.

This worked for me (Prototype, but it looks like MooTools has similar syntax):

var odd = $$('.a').filter(function(item, index) {
    return index % 2 == 0;
});
var even = $$('.a').filter(function(item, index) {
    return index % 2 == 1;
});

Edit: it seems you already covered that in the question, boo on me for answering before reading fully.