code: public FeatureManagerBuilder activationStrategy(ActivationStrategy strategy) {
          if (strategyProvider instanceof DefaultActivationStrategyProvider) {
              ((DefaultActivationStrategyProvider) strategyProvider).addActivationStrategy(strategy);
              return this;
          }
          throw new IllegalStateException("Adding ActivationStrategies is only allowed when using "
              + DefaultActivationStrategyProvider.class.getSimpleName());
      }
docs: Adds an additional {@link ActivationStrategy} to the current {@link ActivationStrategyProvider}. This currently only
  works if you are using the {@link DefaultActivationStrategyProvider}.