gap> M:=ClosedSurface(3);;
gap> Display(IntersectionForm(M));
[ [   0,   0,   1,  -1,  -1,   0 ],
  [   0,   0,   0,   1,   1,   0 ],
  [  -1,   0,   0,   1,   1,  -1 ],
  [   1,  -1,  -1,   0,   0,   1 ],
  [   1,  -1,  -1,   0,   0,   0 ],
  [   0,   0,   1,  -1,   0,   0 ] ]
