code: public Extension getExtension(String urn) {
          if (urn == null || urn.isEmpty()) {
              throw new IllegalArgumentException("urn must be neither null nor empty");
          }
  
          if (!extensions.containsKey(urn)) {
              throw new NoSuchElementException("extension " + urn + " is not available");
          }
  
          return extensions.get(urn);
      }
docs: Provides the {@link Extension} with the given URN
  
  @param urn The URN of the extension
  @return The extension for the given URN
  @throws IllegalArgumentException If urn is null or empty
  @throws NoSuchElementException   If extension with given urn is not available