code: public Configuration configuration(String identity) throws IllegalArgumentException {
      Config variants = configurations.get(identity);
      if (variants == null) {
        return null;
      } else {
        return variants.configuration();
      }
    }
docs: Retrieve the singular configuration for {@code identity}.
  <p>
  If the given identity is associated with multiple variant configurations then an {@code IllegalStateException} will
  be thrown. In this case the {@link #configuration(String, String)} method must be used to select a specific
  variant.
  
  @param identity identity to retrieve
  @return the configuration for the given identity; {@code null} if the identity is not in this configuration
  @throws IllegalArgumentException if the identity is associated with multiple variant configurations