code: func (ks *KeyStore) SignTx(account *Account, tx *Transaction, chainID *BigInt) (*Transaction, error) {
  	if chainID == nil { // Null passed from mobile app
  		chainID = new(BigInt)
  	}
  	signed, err := ks.keystore.SignTx(account.account, tx.tx, chainID.bigint)
  	if err != nil {
  		return nil, err
  	}
  	return &Transaction{signed}, nil
  }
docs: // SignTx signs the given transaction with the requested account.