Definition: Collision sets; copied

For a connected vertex subset $I$ of XIII-C's cycle-vertex table XIIICvertex containing the interaction vertex $y$, with $|I|\ge 2$, ${\Delta }_I=\{x_a=x_b\text{for all }a,b\in I\}$, including intersections and loci after contracting such subsets; diagonals involving only a pre-existing observable subgraph retain their inherited normalization XIIICcoll. Examples on $B_1$: $\{y,u,v\}$, $\{y,v\}$.