[ [    1,   1, -1,   1, -1, 1,-1 ],
  [ -1/4,-1/4,1/4,   0,  0, 0, 0 ],
  [    0,   0,  0,-1/4,1/4, 0, 0 ],
  [    0,   0,  0,   0,  0,-1, 1 ],
  [    0,   0,  0,   0,  0, 1,-1 ] ]