How to expand and collapse parts of NSSplitView programmatically?

I want to replace RBSplitView with NSSplitView in my existing project. The application is now leopard only and I would like to replace RBSplitView with the new NSSplitView shipped with Leopard.

However, I'm missing RBSplitView's handy methods expand and collapse in NSSplitView. How can I expand and collapse parts of NSSplitView programmatically?

I just got programmatic expanding and collapsing of NSSplitView to work. I've also configured my NSSplitView to expand/collapse a subview whenever the divider is double-clicked, so I wanted this to play nice with that feature (and it seems to). This is what I did:

(in this example, splitView is the NSSplitView itself, splitViewSubViewLeft is the subview I wish to expand/collapse and lastSplitViewSubViewLeftWidth is an instance variable of type CGFloat.)

// subscribe to splitView's notification of subviews resizing
// (I do this in -(void)awakeFromNib)
[[NSNotificationCenter defaultCenter]
 addObserver:self
 selector:@selector(mainSplitViewWillResizeSubviewsHandler:)
 name:NSSplitViewWillResizeSubviewsNotification
 object:splitView
 ];

// this is the handler the above snippet refers to
- (void) mainSplitViewWillResizeSubviewsHandler:(id)object
{
    lastSplitViewSubViewLeftWidth = [splitViewSubViewLeft frame].size.width;
}

// wire this to the UI control you wish to use to toggle the
// expanded/collapsed state of splitViewSubViewLeft
- (IBAction) toggleLeftSubView:(id)sender
{
    [splitView adjustSubviews];
    if ([splitView isSubviewCollapsed:splitViewSubViewLeft])
        [splitView
         setPosition:lastSplitViewSubViewLeftWidth
         ofDividerAtIndex:0
         ];
    else
        [splitView
         setPosition:[splitView minPossiblePositionOfDividerAtIndex:0]
         ofDividerAtIndex:0
         ];
}