The Relative neighborhood graph (RNG) is a subgraph
of the Delaunay triangulation consisting of the edges with
empty lune. 
The lunes are also called forbidden regions and must not have input
points inside. A lune of the embedded edge e=(a,b) is defined by the
intersection of the circle c1 with center point(a) and radius e and 
circle c2 with center point(b) and radius e.

The RNG can be used in shape reconstruction algorithms.
