code: func (s *PublicBlockChainAPI) GetBalance(ctx context.Context, address common.Address, blockNr rpc.BlockNumber) (*hexutil.Big, error) {
  	state, _, err := s.b.StateAndHeaderByNumber(ctx, blockNr)
  	if state == nil || err != nil {
  		return nil, err
  	}
  	return (*hexutil.Big)(state.GetBalance(address)), state.Error()
  }
docs: // GetBalance returns the amount of wei for the given address in the state of the
  // given block number. The rpc.LatestBlockNumber and rpc.PendingBlockNumber meta
  // block numbers are also allowed.