code: function (callback) {
          debug('load domain files..');
          self.structureLoader(self.options.domainPath, self.validatorExtension, self.options.useLoaderExtensions, function (err, tree, warns) {
            if (err) {
              return callback(err);
            }
            warnings = warns;
  
            if (!tree || _.isEmpty(tree)) return callback(new Error('No structure loaded for ' + self.options.domainPath + '!'));
  
            self.tree = attachLookupFunctions(tree);
            callback(null);
          });
        }
docs: load domain files...