code: func (r *Store) GetSubStore(name string) (store.Store, error) {
  	if name == "" {
  		return r.store, nil
  	}
  	if sub, found := r.mounts[name]; found {
  		return sub, nil
  	}
  	return nil, errors.Errorf("no such mount point '%s'", name)
  }
docs: // GetSubStore returns an exact match for a mount point or an error if this
  // mount point does not exist