code: public Eh doWith(final Consumer<Configuration> configurer) {
      requireNonNull(configurer, "Configurer callback is required.");
      return doWith((ehconf, conf) -> configurer.accept(ehconf));
    }
docs: Configure callback to manipulate a {@link Configuration} programmatically.
  
  @param configurer A configure callback.
  @return This {@link Eh} cache.