code: func (m *Migrator) migrateStableStore() error {
  	op := "Migrating stable store"
  	m.sendProgress(op, 0, 1)
  
  	total := len(stableStoreKeys)
  	for i, key := range stableStoreKeys {
  		val, err := m.mdbStore.Get(key)
  		if err != nil {
  			if err.Error() != "not found" {
  				return fmt.Errorf("Error getting key '%s': %s", string(key), err)
  			}
  			m.sendProgress(op, i+1, total)
  			continue
  		}
  		if err := m.boltStore.Set(key, val); err != nil {
  			return fmt.Errorf("Error storing key '%s': %s", string(key), err)
  		}
  		m.sendProgress(op, i+1, total)
  	}
  	return nil
  }
docs: // migrateStableStore copies values out of the origin StableStore
  // and writes them into the destination. There are only a handful
  // of keys we need, so we copy them explicitly.