Vera climbs two rungs, balancing a wide brush and a tin, and smiles down at you over her shoulder. "Hold the ladder. Steady. A square like this is forty-eight triangles. One wrong colour and the whole thing sings out of tune. Which colour for the last corner? You choose." The light is going and the square is nearly finished. Hold the ladder, hand me the tin and tell me which colour you want in the middle.