code: def test__reachable(large_collider_graph):
      reachable = nx.algorithms.d_separation._reachable
      g = large_collider_graph
      x = {"F", "D"}
      ancestors = {"A", "B", "C", "D", "F"}
      assert reachable(g, x, ancestors, {"B"}) == {"B", "F", "D"}
      assert reachable(g, x, ancestors, set()) == ancestors
  
imports: from itertools import combinations
  import pytest
  import networkx
label_desc: Use image node_id 28 for calling a global function with example usage: test__reachable(large_collider_graph) without return types