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Figure fig:n21-witnesses: each of the six Holton-McKay duals drawn as its witness Even Level Graph in a radial-by-level layout (source centre, level-k vertices on ring k), coloured by parity. Dashed red edges are the flipped same-parity edges and solid green edges the introduced bridges; applying the switches yields the dual. Duals 1,2 are ELGs outright. draw_witnesses.py generates the combined 2x3 figure and per-dual PNGs from the verified witness JSONs. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>