docs: def add_relations(spec_dict: Mapping[str, Any]) -> Mapping[str, Any]:
      """Add relation keys to spec_dict
  
      Args:
          spec_dict (Mapping[str, Any]): bel specification dictionary
  
      Returns:
          Mapping[str, Any]: bel specification dictionary with added relation keys
      """
  
      # Class 'Mapping' does not define '__setitem__', so the '[]' operator cannot be used on its instances
      spec_dict["relations"]["list"] = []
      spec_dict["relations"]["list_short"] = []
      spec_dict["relations"]["list_long"] = []
      spec_dict["relations"]["to_short"] = {}
      spec_dict["relations"]["to_long"] = {}
  
      for relation_name in spec_dict["relations"]["info"]:
  
          abbreviated_name = spec_dict["relations"]["info"][relation_name]["abbreviation"]
          spec_dict["relations"]["list"].extend((relation_name, abbreviated_name))
          spec_dict["relations"]["list_long"].append(relation_name)
          spec_dict["relations"]["list_short"].append(abbreviated_name)
  
          spec_dict["relations"]["to_short"][relation_name] = abbreviated_name
          spec_dict["relations"]["to_short"][abbreviated_name] = abbreviated_name
  
          spec_dict["relations"]["to_long"][abbreviated_name] = relation_name
          spec_dict["relations"]["to_long"][relation_name] = relation_name
  
      return spec_dict