docs: public static Set<Writable> getUniqueSequence(String columnName, Schema schema,
                                                     SequenceRecordReader sequenceData) {
          int colIdx = schema.getIndexOfColumn(columnName);
          Set<Writable> unique = new HashSet<>();
          while(sequenceData.hasNext()){
              List<List<Writable>> next = sequenceData.sequenceRecord();
              for(List<Writable> step : next){
                  unique.add(step.get(colIdx));
              }
          }
          return unique;
      }