.neigh files

• First line: <# of tetrahedra> <# of nei. per tet (always 4) >
• Remaining lines list of # of neighbors:
  <tetrahedron #> <neighbor1> <neighbor2> <neighbor3> <neighbor4>
  ...

A .neigh file associates with each tetrahedron its neighbors (adjacent tetrahedra), which are indices into the corresponding .ele file. An index of -1 indicates no neighbor (because the tetrahedron is on a boundary of mesh domain). The first neighbor of tetrahedron i is opposite the first corner of tetrahedron i, and so on. It is output by TetGen when -n switch is used.