Dynamic Tweener (caurina)?

programming flash
1

this works;

Tweener.addTween(obj, { _Blur_blurX:10 , time:0});

but I kind of need dynamic tweening more or less like this

var a = "_Blur_blurX"
var b = 10;
Tweener.addTween(obj, { a:b , time:0});

but that doesn’t work… I get a

[Tweener] Error: The property ‘a’ doesn’t seem to be a normal object property of [object MovieClip] or a registered special property.

any suggestions?
t.i.a.
P.