[defrost]

in some form or another is the key to their point.

Here's something to try at home .. exactly your code save for this change:

    i[a] = 1;

... guess what, still compiles, still works !!

WTF ??? you ask, well, you see, X[Y] is just syntactic sugar for X+Y - it's a pointer operation disguised to look like a rose (but it smells just the same).