Regionize guard code

Author: Marco De Salvo

RDFSharp/Model/RDFGraphIndex.cs

@@ -137,9 +137,12 @@ private void Dispose(bool disposing)
     /// </summary>
     internal RDFGraphIndex Add(RDFTriple triple)
     {
-        //Triple (Hash)
+        #region Guards
         if (Hashes.ContainsKey(triple.TripleID))
             return this;
+        #endregion
+
+        //Triple (Hash)
         Hashes.Add(triple.TripleID, new RDFHashedTriple(triple));
 
         //Subject (Register)