code: func (ec *EthereumClient) GetBalanceAt(ctx *Context, account *Address, number int64) (balance *BigInt, _ error) {
  	if number < 0 {
  		rawBalance, err := ec.client.BalanceAt(ctx.context, account.address, nil)
  		return &BigInt{rawBalance}, err
  	}
  	rawBalance, err := ec.client.BalanceAt(ctx.context, account.address, big.NewInt(number))
  	return &BigInt{rawBalance}, err
  }
docs: // State Access
  // GetBalanceAt returns the wei balance of the given account.
  // The block number can be <0, in which case the balance is taken from the latest known block.