code: private String getCategoryTerm(String scheme) {
          List<Category> categories = entry.getCategories(scheme);
          if (categories.isEmpty()) {
              return null;
          } else {
              return categories.get(0).getTerm();
          }
      }
docs: Get the first atom:category term that matches the provided scheme.
  
  @param scheme
  @return the term or null if no match.