code: def naive_bayes_graph():
      """Return a simply Naive Bayes PGM graph."""
      G = nx.DiGraph(name="naive_bayes")
      G.add_edges_from([(0, 1), (0, 2), (0, 3), (0, 4)])
      nx.freeze(G)
      return G
  
imports: from itertools import combinations
  import pytest
  import networkx
label_desc: Use image node_id 4 for calling a global function with example usage: naive_bayes_graph() and returns: G