code: public Configuration configuration(String identity, String variant) {
      Config config = configurations.get(identity);
      if (config == null) {
        return null;
      } else {
        return config.configuration(variant);
      }
    }
docs: Retrieve the singular configuration for {@code identity} and {@code variant}.
  <p>
  If the given identity is associated only with a singular configuration then that configuration will be returned for
  all variants.
  
  @param identity identity to retrieve
  @param variant variant to retrieve
  @return the configuration for the given identity; {@code null} if the identity is not in this configuration
  @throws IllegalArgumentException if the given variant does not exist