code: public void addKeyword(String keyword) {
          if (taxonomy.concepts.containsKey(keyword)) {
              throw new IllegalArgumentException(String.format("Concept %s already exists.", keyword));
          }
  
          taxonomy.concepts.put(keyword, this);
  
          if (synset == null) {
              synset = new TreeSet<>();
          }
  
          synset.add(keyword);
      }
docs: Add a keyword to the concept synset.