code: function (fn) {
      if (!fn || !_.isFunction(fn)) {
        var err = new Error('Please pass a valid function!');
        debug(err);
        throw err;
      }
  
      this.structureLoader = customLoader(fn);
  
      return this;
    }
docs: Inject custom structure loader function
  @param   {Function} fn The function to be injected.
  @returns {Domain}            to be able to chain...