How reset the rotation of a PageItem (or access it's matrix at least) ?

I'd like to apply rotation to a few PageItem instances, but would like to reset the rotation first so it doesn't accumulate when I run the same script multiple times. I'm having a hard time trying to figure out how to restet the transformation matrix.

I've spotted this post which is close to what I need but it's a bit old now.

It looks like there isn't a matrix property anymore.

This:

alert(app.activeDocument.selection[0].matrix);

ouputs undefined

I've tried

app.activeDocument.selection[0].transform(app.getIdentityMatrix());

but it seems transformations are accumulated, multiplied, not reset.

If I want to reset the rotation of a PageItem, how would I go about doing that ?

Thanks,

George