Definition: Collision sets

For each graph in the inventory below, let $y$ be the interaction vertex and let $x,u,v$ denote its displayed observable vertices when present. For a connected vertex subset $I$ containing $y$, with $|I|\ge 2$, let

Include intersections of these loci and the loci obtained after contracting such subsets. Diagonals involving only a pre-existing observable subgraph retain their inherited normalization.