statement to remove multiple MovieClips

I’m having a little trouble because I’m not sure if I’m aware how the for…in statement works.

I want to make a function that will remove every single mc symbol I’ve previously attached to a “mask” movieclip. So i created this:

removeAll = function(){
for(a in _root.mask){

I thought if i do that itll remove each “a” it encounters but I guess I’m missing some part of that “for…in” syntax. Not sure how that works. Anyone help me finish what seems to be an easy task?