I think the key innovation of Leo is it's use of "clones" in outlines (see discussion here; http://leoeditor.com/tutorial-pim.html http://leoeditor.com/tutorial

I think the key innovation of Leo is it's use of "clones" in outlines (see discussion here; http://leoeditor.com/tutorial-pim.html http://leoeditor.com/tutorial-pim.html). In my experience this is the perfect bridge between a strictly hierarchical tree structure and a more generalized graph structure in terms of organizing information. I've played around with other outliners that implement some what similar concepts, but none that I know of implement it as well as Leo.