code: public List<ChallengeHandler> lookup(String location) {
          List<ChallengeHandler> result = Collections.emptyList();
          if (location != null) {
              Node<ChallengeHandler, UriElement> resultNode = findBestMatchingNode(location);
              if (resultNode != null) {
                  return resultNode.getValues();
              }
          }
          return result;
      }
docs: Locate all challenge handlers to serve the given location.
  
  @param location a location
  @return a collection of {@link ChallengeHandler}s if found registered at a matching location or an empty list if none are found.