code: public Set<String> variants(String identity) throws IllegalArgumentException {
      Config config = configurations.get(identity);
      if (config == null) {
        throw new IllegalArgumentException("Identity " + identity + " does not exist.");
      } else {
        return config.variants();
      }
    }
docs: Return the set of variants defined for the given configuration.
  <p>
  If the given identity does not exist then an {@code IllegalArgumentException} is thrown. If the given identity is
  not variant-ed then an empty set is returned.
  
  @return the set of variants; possibly empty.
  @throws IllegalArgumentException if the identity does not exist