question about plot3d



i tried this:

f(x,y):=block([],
if (y< -sqrt(1-x^2)) then return(0),
if (y>sqrt(1-x^2)) then return(0),
return(sin(x*y)));

plot3d(
f(x,y),[x,-1,1], [y, -1,1]
);

But this does no work. I got  graph of function sin(x*y). What is wrong?
> Hello, list.
>
> By using maple it's possible to plot something like this:
>
> plot3d(sin(x*y), x = -1 .. 1, y = -sqrt(1-x^2) .. sqrt(1-x^2), axes = 
> boxed);
>
> Here the area that coordinates x,y are changing in is a circle instead 
> of square.
>
> I tried to do the same with maxima:
> plot3d(sin(x*y),[x,-1,1],[y,-sqrt(1-x^2),sqrt(1-x^2)]);
> But maxima says: plotting: range must be of the form [variable, min, 
> max]; found: [y,-sqrt(1-x^2),sqrt(1-x^2)].
>
> So, is there any way to plot by maxima a function f(x,y) that is 
> bounded by other functions in the x-y plane?
> .