JS CS3 Change geometricBounds

Hi

When I run:
app.selection[0].geometricBounds = [0,0,10,10];
everything works as expected.

I only want to change the width so I tried:
app.selection[0].geometricBounds[3] += 10;

or

app.selection[0].geometricBounds[3] = app.selection[0].geometricBounds[3] + 10;
nothing changes and no error message appears.

I solved it in the following clumsy way eventually:

a = app.selection[0].geometricBounds[0];
b = app.selection[0].geometricBounds[1];
c = app.selection[0].geometricBounds[2];
d = app.selection[0].geometricBounds[3] + 10;

app.selection[0].geometricBounds = [a,b,c,d];

So it finally kind of works but it leaves me scratching my head. Is this really the way to do it?

Thanks for clarification
weller