code: function (callbackError, callbackResult, otherArgs) {
          // forbid to call twice
          if (syncCallback.called) return;
          syncCallback.called = true;
          
          if (callbackError) {
              err = callbackError;
          }
          else if (otherArgs) {
              // Support multiple callback result values
              result = [];
              for (var i = 1, l = arguments.length; i < l; i++) {
                  result.push(arguments[i]);
              }
          }
          else {
              result = callbackResult;
          }
          
          // Resume fiber if yielding
          if (yielded) fiber.run();
      }
docs: Create virtual callback