static void Save(sentence_pair pair, SaveInfo info)
        {
            string fileName = info.FileName + ".txt";

            if (!File.Exists(fileName))
            {
                // Create a file to write to.
                using (StreamWriter sw = File.CreateText(fileName))
                {
                    sw.WriteLine("<?xml version = \"1.0\" encoding = \"utf - 8\" ?>");
                    sw.WriteLine("<sentence_pairs>");
                }
            }


            // This text is always added, making the file longer over time
            // if it is not deleted.
            using (StreamWriter sw = File.AppendText(fileName))
            {
                sw.WriteLine($"<sentence_pair classification = \"\" edit_distance = \"{pair.edit_distance}\" shared_words = \"{pair.shared_words}\" pair_no = \"{info.Counter}\" >");

                sw.WriteLine($"<sentence_1>{pair.sentence_1}</sentence_1>");

                sw.WriteLine($"<sentence_2>{pair.sentence_2}</sentence_2>");

                sw.WriteLine($"</sentence_pair>");
            }
        }
 static void SavePairToFile(sentence_pair pair, SaveInfo info)
 {
     lock (info.lock_obj)
     {
         if (info.Counter <= info.MaxSentencePairs)
         {
             Save(pair, info);
             info.Increment();
         }
     }
 }