Neighbours consists of the object identifiers of the four adjacent tetrahedra, listed such that the first is opposite pointA, the second is opposite pointB, the third is opposite pointC and the fourth is opposite pointD.
The four points defining the vertices of a tetrahedron are provided by pointA, pointB, pointC, and pointD. The 0 to four neighbouring tetrahedra are given by the attribute neighbours. (The relationship between the four points and the neighbours of a tetrahedron, as stated in the constraints clause above, is equivalent to that between the three points and the neighbours of a triangle. See the diagram under Triangle for further clarification.)
The four faces of the tetrahedron are assumed to be triangles defined by straight line segments; thus, neither the triangles nor the line segments need be specified explicitly. If one or more neighbours do not exist, this may be indicated by the use of the nil value.