glycerine

// how can we do a tree in Rust, where each node
// tells its children to update, and in turn, the
// child may update the parent?
//
// The following example illustrates the technique. Notice how we
//  can't let the children have full backpointers to
//  parent nodes, and instead have to have them point
//  back to parent.data.  Otherwise the two paths 
//  (parent) and (parent->child) overlap, and modifying
//  parent could unsafely clobber (parent->child) use.