Toggle Tree in Source

Set TREE=Toggle acts on both panes of a dual lister. The command can be made to act on Left or Right pane - e.g. Set TREE=Toggle, Left. Is there a way of making it act on the current source pane, which could be left or right, and ignore the other pane?

Regards, AB

I can't think of a good workaround so I filed a feature request.