code: def fork_graph():
      """Return a three node fork graph."""
      G = nx.DiGraph(name="fork")
      G.add_edges_from([(0, 1), (0, 2)])
      nx.freeze(G)
      return G
  
imports: from itertools import combinations
  import pytest
  import networkx
label_desc: Use image node_id 2 for calling a global function with example usage: fork_graph() and returns: G