docs: public static Map<String,Set<Writable>> getUniqueSequence(List<String> columnNames, Schema schema,
                                                                 SequenceRecordReader sequenceData) {
          Map<String,Set<Writable>> m = new HashMap<>();
          for(String s : columnNames){
              m.put(s, new HashSet<>());
          }
          while(sequenceData.hasNext()){
              List<List<Writable>> next = sequenceData.sequenceRecord();
              for(List<Writable> step : next) {
                  for (String s : columnNames) {
                      int idx = schema.getIndexOfColumn(s);
                      m.get(s).add(step.get(idx));
                  }
              }
          }
          return m;
      }