hmm, good question.
The structures currently have a littel arrow thingie that folds and unfolds, maybe that one could be expanded with somesort of "set nil". Or maybe right click on the structure bar itself and set it?
However it's nice to have it visible that something is set to nil. Either a checkbox or maybe a colour change?
I'm not a GUI programemr myself so I don't knwo what options you have to choose from.
My best suggestion is to use right-click to set/unset nil, and show it with a colour change. Would that be easier than making checkboxes..?
PS: I just noticed my skype was turned off. catch me there, it might be quicker than forum posts
