code: function cacheClient (cacheClient) {
      if (defined(cacheClient)) {
          // validate that cache client has required methods
          requireValidCacheClient(cacheClient)
          // set default
          immutable.defaultCacheClient = cacheClient
          // return immutable
          return ImmutableCore
      }
      // return default value
      return immutable.defaultCacheClient
  }
docs: @function cacheClient
  
  get/set the default cache client
  
  @param {object} cacheClient
  
  @returns {ImmutableCore|object}
  
  @throws {Error} on invalid cacheClient