//
			//  Path Browsing facilities
			//   
			public IEnvironmentVariableFilePath GetSisterFilePath(string fileName)
			{
				Debug.Assert(fileName != null); // Enforced by contract
				Debug.Assert(fileName.Length > 0); // Enforced by contract
				var path = PathBrowsingHelpers.GetSisterFileWithName(this, fileName);
				var pathTyped = path as IEnvironmentVariableFilePath;
				Debug.Assert(pathTyped != null);
				return pathTyped;
			}
            //
            //  Path Browsing facilities
            //
            public IAbsoluteFilePath GetSisterFileWithName(string fileName)
            {
                Debug.Assert(fileName != null);                 // Enforced by contract
                Debug.Assert(fileName.Length > 0);              // Enforced by contract
                var path      = PathBrowsingHelpers.GetSisterFileWithName(this, fileName);
                var pathTyped = path as IAbsoluteFilePath;

                Debug.Assert(pathTyped != null);
                return(pathTyped);
            }