code: func (b *SimulatedBackend) CodeAt(ctx context.Context, contract common.Address, blockNumber *big.Int) ([]byte, error) {
  	b.mu.Lock()
  	defer b.mu.Unlock()
  
  	if blockNumber != nil && blockNumber.Cmp(b.blockchain.CurrentBlock().Number()) != 0 {
  		return nil, errBlockNumberUnsupported
  	}
  	statedb, _ := b.blockchain.State()
  	return statedb.GetCode(contract), nil
  }
docs: // CodeAt returns the code associated with a certain account in the blockchain.