Referring to linked clips

How can I instantiate a linked clip using a variable name?

In other words, I want to say

"var someName:String = ‘clipName’;
var someClip:MovieClip = addChild (new someName as MovieClip);

But obviously that throws an error, since the someName is a string, not a movieclip. In previous versions, I could use the “eval” function. AS3 uses casting to force someName to be a movieclip, as long as it is a subclass of movieclip, but in this case, I can’t do that.