http://takingnotenow.blogspot.com/2007/12/luhmanns-zettelkasten.html
Luhmann's original Zettelkasten numbering system is intriguing! A little like Ted Nelson's Xanadu Tumblers, maybe @enkiv2 ?
<<Thus, when he felt that a certain term needed to be further discussed or the information about it needed to be supplemented, he would begin a new slip that addded a letter, like a, b, or c to the number. So, a branching from slip 1/6 could have branches like 1/6a or 1/6b, up to 1/6z. >>