code: def test_unknown_causal_model_relationship_is_undefined():
      causal_model = create_causal_model_from_equations(
          """
      A = norm(loc=0,scale=0.1)
      B = norm(loc=0, scale=0.1)
      Y = 0.5*B + 2*A+ norm(loc=0, scale=0.1)
      Z->Y,A
      """
      )
      assert "Z" in causal_model.graph.nodes
      try:
          mech = causal_model.causal_mechanism("Z")
          raise AssertionError(
              "The causal mechanism is defined for unknown model node!"
          )
      except KeyError as ke:
          pass
  
file_path: dowhy/tests/gcm/test_equation_parser.py
imports: import networkx
  import numpy
  import pandas
  from flaky import flaky
  from dowhy.gcm import AdditiveNoiseModel, EmpiricalDistribution, ProbabilisticCausalModel, create_causal_model_from_equations, fit
  from dowhy.gcm.causal_mechanisms import ConditionalStochasticModel
  from dowhy.gcm.ml import create_linear_regressor_with_given_parameters
label_desc: Use image node_id 3 for calling a global function with example usage: test_unknown_causal_model_relationship_is_undefined() without return types