<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def generic_var(self, key, value=None):
 """ Stores generic variables in the session prepending it with _GENERIC_VAR_KEY_PREFIX. """

        return self._get_or_set('{0}{1}'.format(self._GENERIC_VAR_KEY_PREFIX, key), value)