docs: private static Selector DNF0(Selector tree) {
      if (!(tree instanceof Operator))
        return tree;
      Operator oper = (Operator) tree;
      switch(oper.getOp()) {
      case Selector.AND:
        return processAND(oper.getOperands()[0], oper.getOperands()[1]);
      case Selector.OR:
        return makeOR(DNF0(oper.getOperands()[0]), DNF0(oper.getOperands()[1]));
      default:
        return tree;
      }
    }