<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def rate(self, currency):
 """Return quotation between the base and another currency"""

        if not self._backend:
            raise ExchangeBackendNotInstalled()
        return self._backend.rate(currency)