我正在使用Deezer SDK for ios，我不会向Radio Deezer提出请求，所以这就是我所做的

- (void)requestRadioForListening:(NSString*)radioId {
    NSString* servicePath = [NSString stringWithFormat:@"%@/%@/%@", @"radio", radioId, @"tracks"];
    NSDictionary* params = [NSDictionary dictionaryWithObject:@"true" forKey:@"next"];
    DeezerRequest* request = [_deezerConnect createRequestWithServicePath:servicePath
                                                                   params:params
                                                                 delegate:self];
    [_deezerConnect launchAsyncRequest:request];
}

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现在我想播放前一首曲目。我怎么做？要添加什么参数才能获得前一首曲目？