// run tx; the tx message handler
  runTx (tx, environment = new Environment()) {
    this.environment = environment

    if (Buffer.isBuffer(tx) || typeof tx === 'string') {
      tx = new Transaction(tx)
      if (!tx.valid) {
        throw new Error('Invalid transaction signature')
      }
    }

    // look up sender
    let fromAccount = this.environment.state.get(tx.from.toString())
    if (!fromAccount) {
      throw new Error('Sender account not found: ' + tx.from.toString())
    }

    if (fromAccount.get('nonce').gt(tx.nonce)) {
      throw new Error(`Invalid nonce: ${fromAccount.get('nonce')} > ${tx.nonce}`)
    }

    fromAccount.set('nonce', fromAccount.get('nonce').add(new U256(1)))

    let isCreation = false

    // Special case: contract deployment
    if (tx.to.isZero() && (tx.data.length !== 0)) {
      console.log('This is a contract deployment transaction')
      isCreation = true
    }

    // This cost will not be refunded
    let txCost = 21000 + (isCreation ? 32000 : 0)
    tx.data.forEach((item) => {
      if (item === 0) {
        txCost += 4
      } else {
        txCost += 68
      }
    })

    if (tx.gasLimit.lt(new U256(txCost))) {
      throw new Error(`Minimum transaction gas limit not met: ${txCost}`)
    }

    if (fromAccount.get('balance').lt(tx.gasLimit.mul(tx.gasPrice))) {
      throw new Error(`Insufficient account balance: ${fromAccount.get('balance').toString()} < ${tx.gasLimit.mul(tx.gasPrice).toString()}`)
    }

    // deduct gasLimit * gasPrice from sender
    fromAccount.set('balance', fromAccount.get('balance').sub(tx.gasLimit.mul(tx.gasPrice)))

    const handler = isCreation ? this.createHandler.bind(this) : this.callHandler.bind(this)
    let ret = handler({
      to: tx.to,
      from: tx.from,
      gasLimit: tx.gasLimit - txCost,
      value: tx.value,
      data: tx.data
    })

    // refund unused gas
    if (ret.executionOutcome === 1) {
      fromAccount.set('balance', fromAccount.get('balance').add(tx.gasPrice.mul(ret.gasLeft.add(ret.gasRefund))))
    }

    // save new state?

    return {
      executionOutcome: ret.executionOutcome,
      accountCreated: isCreation ? ret.accountCreated : undefined,
      returnValue: isCreation ? undefined : ret.returnValue,
      gasLeft: ret.gasLeft,
      logs: ret.logs
    }
  }