def _traverse(self, n=1):
        """Traverse state history. Used by `back` and `forward` methods.

        :param int n: Cursor increment. Positive values move forward in the
            browser history; negative values move backward.

        """
        if not self.history:
            raise exceptions.RoboError('Not tracking history')
        cursor = self._cursor + n
        if cursor >= len(self._states) or cursor < 0:
            raise exceptions.RoboError('Index out of range')
        self._cursor = cursor
Traverse state history. Used by `back` and `forward` methods.

        :param int n: Cursor increment. Positive values move forward in the
            browser history; negative values move backward.

        
robobrowser/browser.py