code: func SwitchLocalAccount(ctx context.Context, accountID string) (context.Context, error) {
  	if la := GetLocalAuth(ctx); la != nil {
  		if la.DefaultAccountID == accountID {
  			return ctx, nil
  		}
  		for _, acc := range la.Accounts {
  			if acc.ID == accountID {
  				la.DefaultAccountID = accountID
  				return SetLocalAuth(ctx, la), nil
  			}
  		}
  	}
  	return nil, ErrNoLocalAuthAccount
  }
docs: // SwitchLocalAccount changes default logical account selected in the context.
  //
  // For example, it can be used to switch the context into using "system" account
  // by default. The default account is transparently used by LUCI-aware tools.
  //
  // If the requested account is available, modifies LUCI_CONTEXT["local_auth"]
  // in the context and returns the new modified context.
  //
  // If the given account is already default, returns the context unchanged.
  //
  // If the given account is not available, returns (nil, ErrNoLocalAuthAccount).