* @return Proposition
   */
  public static Proposition retractRule(Stella_Object rulename) {
    { Proposition proposition = Logic.getRule(rulename);

      if (proposition == null) {
        return (null);
      }
      Proposition.updatePropositionTruthValue(proposition, Logic.KWD_RETRACT_TRUE);
      return (proposition);
    }
  }