code: public RetryPolicy<R> withDelay(DelayFunction<R, ? extends Throwable> delayFunction) {
      Assert.notNull(delayFunction, "delayFunction");
      this.delayFn = delayFunction;
      return this;
    }
docs: Sets the {@code delayFunction} that computes the next delay before retrying.
  
  @param delayFunction the function to use to compute the delay before a next attempt
  @throws NullPointerException if {@code delayFunction} is null
  @see DelayFunction