code: def test_add_node():
      graph = DiGraph()
      graph.add_node("A")
      assert "A" in graph
  
file_path: baikal/tests/_core/test_digraph.py
imports: import pytest
  from baikal._core.digraph import DiGraph, NodeNotFoundError, CyclicDiGraphError
label_desc: Use image node_id 1 for calling a global function with example usage: test_add_node() without return types