code: def identify_mediation(
      graph: nx.DiGraph,
      action_nodes: List[str],
      outcome_nodes: List[str],
  ):
      """Find a valid mediator if it exists.
  
      Currently only supports a single variable mediator set.
      """
      mediation_var = None
      mediation_paths = get_all_directed_paths(
          graph, action_nodes, outcome_nodes
      )
      eligible_variables = get_descendants(graph, action_nodes) - set(
          outcome_nodes
      )
      # For simplicity, assuming a one-variable mediation set
      for candidate_var in eligible_variables:
          is_valid_mediation = check_valid_mediation_set(
              graph,
              action_nodes,
              outcome_nodes,
              parse_state(candidate_var),
              mediation_paths=mediation_paths,
          )
          logger.debug(
              "Candidate mediation set: {0}, on_mediating_path: {1}".format(
                  candidate_var, is_valid_mediation
              )
          )
          if is_valid_mediation:
              mediation_var = candidate_var
              break
      return parse_state(mediation_var)
  
imports: import itertools
  import logging
  from enum import Enum
  from typing import Dict, List, Optional, Union
  import networkx
  import sympy
  import sympy.stats
  from dowhy.causal_identifier.efficient_backdoor import EfficientBackdoor
  from dowhy.causal_identifier.identified_estimand import IdentifiedEstimand
  from dowhy.graph import check_dseparation, check_valid_backdoor_set, check_valid_frontdoor_set, check_valid_mediation_set, do_surgery, get_all_directed_paths, get_backdoor_paths, get_descendants, get_instruments, has_directed_path
  from dowhy.utils.api import parse_state
label_desc: Use image node_id 12 for calling a global function with example usage: identify_mediation(graph, action_nodes, outcome_nodes) and returns: parse_state