code: func (r *Store) Prune(ctx context.Context, tree string) error {
  	for mp := range r.mounts {
  		if strings.HasPrefix(mp, tree) {
  			return errors.Errorf("can not prune subtree with mounts. Unmount first: `gopass mounts remove %s`", mp)
  		}
  	}
  
  	ctx, store, tree := r.getStore(ctx, tree)
  	return store.Prune(ctx, tree)
  }
docs: // Prune will remove a subtree from the Store