def get_nodes_count(bpmn_graph, node_type=None):
return len(bpmn_graph.get_nodes(node_type=node_type))<EOL>
Gets the count of nodes of the requested type.
If no type is provided,
the count of all nodes in BPMN diagram graph is returned.

:param bpmn_graph: an instance of BpmnDiagramGraph representing BPMN model.
:param node_type: string with valid BPMN XML tag name
                  (e.g. 'task', 'sequenceFlow').