Labelled indices on arrays



Dear all,
Sorry for this very simple question, I basically would like to use
arrays to create a bunch of repetitive variables.

I would like to create variables for each of the 2x2 possible states of
my model. Along the first dimension, the state is either a or b. Along
the second: x or y.

I started by creating manually:
	variable_ax: 1;
	variable_ay: 33;
	variable_bx: 4;
	variable_by: 50;

What I would like to do is something like being able to assign:
	variable['a]['x] : 1;
	variable['a]['y] : 33;

and so on. But i get an error (assignment: cannot assign to
(variable['a])['x]).
In other words, I would like to create arrays the indices of which are
actually letters, not numbers.

I looked and looked at the manual but arrays are a bit cryptic to me.
Could anyone explain to me briefly how I could achieve that?

Thanks for your help!

Patrick