Changing the barlines surrounding a selection

Hi, I'm having trouble changing the barlines surrounding a range selection.

I've combed through some old forum posts and doxygen several times, but I've never gotten it to work exactly how I would like (either the wrong barline changes, or nothing changes at all)...

    function addRepeats() {
        var c = curScore.newCursor();
        //c.filter = Segment.All;
        c.rewind(Cursor.SELECTION_START);
        //var seg = curScore.selection.startSegment
        //while (seg.prev && seg.segmentType != Segment.BarLineType) {
        //  seg = seg.prev
        //}
        //while(! seg.elementAt(0).barlineType) {
        //  seg.prev
        //}
        /*while(c.prev() && ! c.segment.elementAt(0).barlineType) {
            c.prev();
            var seg = c.segment;
        }*/

    curScore.selection.select(c) //tackle weird range select problem
    while(curScore.selection.elements[0] && curScore.selection.elements[0].type != Element.BARLINE) {
        cmd('prev-element')
    }
    curScore.selection.elements[0].barlineType = 4;
        //var seg = c.measure.elements[0];
        //seg.barlineType = 4;
        //seg.elementAt(0).barlineType = 4;
    }

Currently I'm only trying to get the leftmost barline to work, and the stuff in comments is other things I've tried.

Thanks in advance for your time :)