code: private <X extends YamlContainerNode> X putContainer(
              YamlNode key, X value) {
          put(key, value);
          return value;
      }
docs: Puts the specified container under the specified key and return
  {@code this}.
  
  @param <X>   the container type
  @param key   the key
  @param value the container
  
  @return {@code this}