Group fill opacity

Hi.

I'm trying to set the opacity and fillOpacity of all layers and groups to 100% (an later also store the previous values in an array and restore them when done).

This works fine for the artLayers but the layerSet object doesn't seem to have a fillOpacity property, yet I can set a fillOpacity value on a group manually.

So how do I set and retrieve the fillOpacity value of a group?