ok   Euler: E = 4n^2 and F = E - V + 2 faces traced (n <= 25)
ok   exactly one unbounded face; n^2 + (n-1)^2 bounded faces
ok   every bounded face is a 4-cycle spanning the unit square centred at (i,j), |i|+|j| <= n-1; all of these occur
ok   every edge lies in the 2x2 block named in the proof (column x - sgn x / row y - sgn y)
ok   Problem 12 signs: exactly one -1 on every bounded face (product -1)
ok   alternate-columns and Kasteleyn-Wilson signs: product -1 on every bounded face (Kasteleyn signings)
ok   control: -1 on the vertical dominoes of every other ROW is not a Kasteleyn signing (product +1 on every face)
TOTAL: 0 failed  (0.5 s)
