• the realization that the set operations intersection, noncontained difference, and nondisjoint union produce consecutive sets from consecutive sets and can be used to enrich the input set collection C into a normalized,
complete collection C. The PQR tree associated to C is unique up
to equivalence and depends only on C.
• the realization that the concept of orthogonality between sets can be
used to characterize the nodes that appear in a PQR tree for C, through
the collection C⊥, which holds every set orthogonal to C.