/// <summary> 
 /// For all DirectedEdges in result, form them into MaximalEdgeRings.
 /// </summary>
 /// <param name="dirEdges"></param>
 /// <returns></returns>
 private IList BuildMaximalEdgeRings(IEnumerable dirEdges)
 {
     IList maxEdgeRings = new ArrayList();
     for (IEnumerator it = dirEdges.GetEnumerator(); it.MoveNext(); )
     {
         DirectedEdge de = (DirectedEdge)it.Current;
         if (!de.IsInResult || !de.Label.IsArea()) continue;
         // if this edge has not yet been processed
         if (de.EdgeRing != null) continue;
         MaximalEdgeRing er = new MaximalEdgeRing(de, _geometryFactory);
         maxEdgeRings.Add(er);
         er.SetInResult();
     }
     return maxEdgeRings;
 }
        /// <summary>
        /// Form <see cref="DirectedEdge" />s in graph into Minimal EdgeRings.
        /// (Minimal Edgerings must be used, because only they are guaranteed to provide
        /// a correct isHole computation).
        /// </summary>
        /// <param name="dirEdges"></param>
        /// <returns></returns>
        private IList BuildEdgeRings(IList dirEdges)
        {
            IList edgeRings = new ArrayList();
            foreach (DirectedEdge de in dirEdges)
            {
                // if this edge has not yet been processed
                if (de.IsInResult && de.EdgeRing == null)
                {
                    MaximalEdgeRing er = new MaximalEdgeRing(de, _geometryFactory);

                    er.LinkDirectedEdgesForMinimalEdgeRings();
                    IList minEdgeRings = er.BuildMinimalRings();
                    foreach(object o in minEdgeRings)
                        edgeRings.Add(o);
                }
            }
            return edgeRings;
        }