code: def export_diagram_plane_elements(root, diagram_attributes, plane_attributes):
          
          diagram = eTree.SubElement(root, BpmnDiagramGraphExport.bpmndi_namespace + "BPMNDiagram")
          diagram.set(consts.Consts.id, diagram_attributes[consts.Consts.id])
          diagram.set(consts.Consts.name, diagram_attributes[consts.Consts.name])
  
          plane = eTree.SubElement(diagram, BpmnDiagramGraphExport.bpmndi_namespace + "BPMNPlane")
          plane.set(consts.Consts.id, plane_attributes[consts.Consts.id])
          plane.set(consts.Consts.bpmn_element, plane_attributes[consts.Consts.bpmn_element])
  
          return diagram, plane
docs: Creates 'diagram' and 'plane' elements for exported BPMN XML file.
          Returns a tuple (diagram, plane).
  
          :param root: object of Element class, representing a BPMN XML root element ('definitions'),
          :param diagram_attributes: dictionary that holds attribute values for imported 'BPMNDiagram' element,
          :param plane_attributes: dictionary that holds attribute values for imported 'BPMNPlane' element.