Issues in DAG.cpdag()

[kenneth-lee-ch]

I tried the following code

from graphical_models.classes.dags.dag import DAG

arcs = {(0, 5), (0, 8), (2, 0),(3, 6),(5, 4),(5, 7),(6, 0),(7, 1),(7, 4),(9, 5),(9, 6),(9, 8)}
g = DAG(arcs=arcs)
cg = g.cpdag()
print(cg.edges)

Then, I get the following

{frozenset({4, 7})}

According to my understanding, .edges returns all undirected edges in a CPDAG. How come this is the case? I also see that cg.arcs does not contain (7, 4).