public override IRelationship CreateRelationship()
        {
            var relationshipElement = new XElement(TAG_RELATIONSHIP);
            var relationship        = new XMindRelationship(relationshipElement, this);

            _adaptableRegistry.RegisterByNode(relationship, relationship.Implementation);
            return(relationship);
        }
        public override ISheet CreateSheet()
        {
            var sheetElement = new XElement(TAG_SHEET);
            // GetWorkbookElement().Add(sheetElement);
            XMindSheet sheet = new XMindSheet(sheetElement, this);

            _adaptableRegistry.RegisterByNode(sheet, sheet.Implementation);
            return(sheet);
        }