[ [ -1, 0, 0, 0 ],
  [  1,-1, 0, 0 ],
  [  0, 0, 0, 0 ],
  [  0, 1,-1, 0 ],
  [  0, 0, 0, 0 ],
  [  0, 0, 0, 0 ],
  [  0, 0, 2,-1 ] ]