gap> B:=[ [ 0, 1, -1, -1, 1, 0 ],
>         [ 1, 0, 1, 1, 0, 0 ],
>         [ 0, 0, 1, 0, 0, 0 ],
>         [ 0, 0, 0, 1, -1, 0 ],
>         [ 0, 0, 1, 1, 0, 0 ],
>         [ 0, 0, 1, 1, 0, 1 ] ];;
gap> Display(IntersectionForm(M,B));
[ [   0,   1,   0,   0,   0,   0 ],
  [  -1,   0,   0,   0,   0,   0 ],
  [   0,   0,   0,   0,   1,   0 ],
  [   0,   0,   0,   0,   0,   1 ],
  [   0,   0,  -1,   0,   0,   0 ],
  [   0,   0,   0,  -1,   0,   0 ] ]
