All triangles have three adjacent triangles sharing a common (implicitly defined) arc.
This is a subclass for the description of triangular irregular networks that may be used to define an enclosing surface in 3 dimensional space. Note that every triangle will have three triangles as adjacent neighbours; that is, no triangle will have nil as a neighbour. Also, the surface defined by an instance of TINEnvelope is multivalued, having more than a single z value for any x,y location (except for points on the periphery).