In my last post, I dissociated the concept of "outlineness" from a
graph, and showed at least the skeleton of a data structure that allows
the power of graphs while preserving the nature of an outline.
In this post, we will fix a flaw in the model built up to that
point, which is that there is no way to obtain a list of parents,
given a node, only a list of children. For various reasons, this is
necessary to building an outliner, so this flaw must be fixed.