RelationSet

A tree, fully decomposed into Pcs and Content relations - Lindholm's representation of a tree as a set (section 2.3), which is what let his merge algorithm be defined as set operations (union, difference) rather than a tree-editing procedure. This is the foundation for an eventual MERGE.md-style merge that (unlike today's attribute-only EditDetector/Merger) can also detect and reconcile structural edits: moves and reordering (Kaining's 03-single-move, 05-move-move-divergent, 09-zorder cases).