code: def asia_graph():
      """Return the 'Asia' PGM graph."""
      G = nx.DiGraph(name="asia")
      G.add_edges_from(
          [
              ("asia", "tuberculosis"),
              ("smoking", "cancer"),
              ("smoking", "bronchitis"),
              ("tuberculosis", "either"),
              ("cancer", "either"),
              ("either", "xray"),
              ("either", "dyspnea"),
              ("bronchitis", "dyspnea"),
          ]
      )
      nx.freeze(G)
      return G
  
imports: from itertools import combinations
  import pytest
  import networkx
label_desc: Use image node_id 5 for calling a global function with example usage: asia_graph() and returns: G