private ReadOnlyCollection <Vector2> GenerateDeadEnd(HalfEdge <IVertexBuilder, IHalfEdgeBuilder, IFaceBuilder> a)
{
Contract.Requires(a != null);
//Get the builder for the edge ending at _vertex
var b = a.BuilderEndingWith(_vertex);
var ld = b.Left.ClosestPointDistanceAlongLine(_vertex.Position);
b.LeftEnd = b.Left.Position + b.Left.Direction * ld;
var rd = b.Right.ClosestPointDistanceAlongLine(_vertex.Position);
b.RightEnd = b.Right.Position + b.Right.Direction * rd;
//Dead ends do not create *any* junction, so return null for the junction shape
return(null);
}