code: @Override
      public Map<Integer, String> extract(final String text) {
          Set<String> tmpKwd = new LinkedHashSet<>(generateTokenizer().tokenize(text));
          
          Map<Integer, String> keywordSequence = new LinkedHashMap<>();
          
          int position = 0;
          for(String keyword : tmpKwd) {
              keywordSequence.put(position, keyword);
              ++position;
          }
          
          return keywordSequence;
      }
docs: This method gets as input a string and returns as output a numbered sequence
  of the unique tokens. In the returned map as keys we store the position of the word
  in the original string and as value the actual unique token in that position.
  Note that the sequence includes only the position of the first occurrence of
  each word while the next occurrences are ignored.
  
  @param text
  @return