atan2 again



Dear all,

atan2 (0,x) is 0 when x>0 and x constant
but atan2(0,a^2-b) is not evaluated when a^2-b>0 and a,b constant:
(but it is evaluated when a,b are not declared constant) 

atan2(0,x);
atan2(0,x)
assume(x>0);
[x>0]
atan2(0,x);
0
declare(x,constant);
done
atan2(0,x);
0

atan2(0,a^2-b);
atan2(0,a^2-b)
assume(a^2-b>0);
[a^2>b]
atan2(0,a^2-b);
0
declare(a,constant);
done
declare(b,constant);
done
atan2(0,a^2-b);
atan2(0,a^2-b)

How can I make this work?

Best, Nyso