docs: def context(self, id):
          """Return a circuit breaker for the given ID."""
          if id not in self.circuits:
              self.circuits[id] = self.factory(self.clock, self.log.getChild(id),
                                               self.error_types, self.maxfail,
                                               self.reset_timeout,
                                               self.time_unit,
                                               backoff_cap=self.backoff_cap,
                                               with_jitter=self.with_jitter)
          return self.circuits[id]