For tree data you may want to look into various XML designers / editors. You can easily define a recursive XSL style sheet that has the structure you want, after that there are various editors that automatically read that structure and give you a GUI from it.
It's been a few years but I found XMLSpy to be fairly decent. http://www.altova.com/xmlspy.html
Edit: The simplest tree editor is probably the file system on your computer. However, the problem is for proof's I suspect your going to want to have some sort of graph structure.
PS: If your willing to do some light coding XQuery/XSL-FO let's you transform the raw XML into various more user friendly formats like PDF's. However, if your structure is really nested that quickly get's hard for people to understand.
Edit: The simplest tree editor is probably the file system on your computer. However, the problem is for proof's I suspect your going to want to have some sort of graph structure.
PS: If your willing to do some light coding XQuery/XSL-FO let's you transform the raw XML into various more user friendly formats like PDF's. However, if your structure is really nested that quickly get's hard for people to understand.