code: @SuppressWarnings("unchecked")
      public static List<HierarchicalConfiguration> getRequiredConfigurationsAt(HierarchicalConfiguration config,
                                                                                String key) throws
          DeployerConfigurationException {
          List<HierarchicalConfiguration> configs = getConfigurationsAt(config, key);
          if (CollectionUtils.isEmpty(configs)) {
              throw new MissingConfigurationPropertyException(key);
          } else {
              return configs;
          }
      }
docs: Returns the sub-configuration tree whose root is the specified key. If the tree is missing a
  {@link MissingConfigurationPropertyException} is thrown
  
  @param config the configuration
  @param key    the key of the configuration tree
  @return the sub-configuration tree, or null if not found
  @throws MissingConfigurationPropertyException if the property is missing from the configuration
  @throws DeployerConfigurationException        if an error occurs