code: def collider_graph():
      """Return a collider/v-structure graph with three nodes."""
      G = nx.DiGraph(name="collider")
      G.add_edges_from([(0, 2), (1, 2)])
      nx.freeze(G)
      return G
  
imports: from itertools import combinations
  import pytest
  import networkx
label_desc: Use image node_id 3 for calling a global function with example usage: collider_graph() and returns: G