def bpmn_diagram_to_dot_file(bpmn_diagram, file_name):
g = bpmn_diagram.diagram_graph<EOL>write_dot(g, file_name + "<STR_LIT>")<EOL>
Convert diagram graph to dot file

:param bpmn_diagram: an instance of BPMNDiagramGraph class,
:param file_name: name of generated file.