docs: def model_to_pdag(model):
          """Construct the DAG pattern (representing the I-equivalence class) for
          a given DAG. This is the "inverse" to pdag_to_dag.
          """
  
          if not isinstance(model, DAG):
              raise TypeError("model: Expected DAG instance, " +
                              "got type {model_type}".format(model_type=type(model)))
  
          skel, separating_sets = ConstraintBasedEstimator.build_skeleton(
                                      model.nodes(),
                                      model.get_independencies())
          pdag = ConstraintBasedEstimator.skeleton_to_pdag(skel, separating_sets)
  
          return pdag