student.add_edges(("diff", "intel"), ("grades",))
print(sorted(student.edges()))
student.add_states("diff", ("hard", "easy"))
print([m for m in student.states("diff")])
student.add_states("intel", ("smart", "avg", "dumb"))
print([m for m in student.states("intel")])
student.add_states("grades", ("A", "B", "C"))