seed 6: first failure at trial 302 (n=21, k=4): a cycle of Delta has non-positive length at vertex 17
arcs [(0, 4), (0, 8), (0, 9), (0, 13), (0, 14), (0, 20), (1, 4), (1, 5), (1, 8), (1, 10), (1, 18), (2, 4), (2, 5), (3, 4), (3, 5), (3, 6), (3, 7), (3, 10), (3, 11), (3, 13), (3, 19), (4, 6), (4, 8), (4, 12), (4, 16), (4, 17), (4, 19), (5, 6), (5, 7), (5, 8), (5, 9), (5, 10), (5, 13), (5, 15), (6, 7), (8, 10), (8, 11), (8, 13), (8, 14), (8, 15), (8, 18), (10, 12), (10, 14), (10, 15), (12, 16), (13, 14), (14, 15), (14, 19), (15, 17), (15, 18), (15, 19), (15, 20), (19, 20)]
roots [0, 1, 2, 3] sets [[0, 4, 6, 7, 8, 9, 10, 13, 14, 15, 18, 19, 20], [1, 4, 5, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 19], [2, 4, 5, 6, 7, 8, 10, 12, 13, 14, 15, 16, 17, 19, 20], [3, 4, 5, 6, 7, 8, 10, 11, 13, 14, 15, 17, 18, 19, 20]]
failing vertex 17 ; J(v) = [2, 3] ; tails of the minimum-cost assignment {2: 4, 3: 15}
   arc of Delta 2 -> 3, length -178572/3000001
   arc of Delta 3 -> 2, length 178572/3000001
   cycle (2, 3) has length 0   <-- exactly 0: a tie
   admissible assignment with tails [4, 15] has cost -3523897/48000016
   admissible assignment with tails [15, 4] has cost -3523897/48000016
   f(4) = {0: '714871/3000001', 1: '940146/3000001', 2: '275794/3000001', 3: '97222/3000001'}
   f(15) = {0: '-2736837/48000016', 1: '148809/48000016', 2: '-5079449/48000016', 3: '-7936601/48000016'}
same instance, same (old) code, 50 other seeds: construction succeeded and was verified in 50 cases
