Proxy Contracts (EIP-17)

{
  // dApp-specific part ensuring that user will receive what he is paying for
  val properFundUsage = {
    val userOut = OUTPUTS(1)
    userOut.propositionBytes == fromBase64("\$userAddress") && // user must be the recipient
      userOut.tokens(0)._1 == fromBase64("\$scTokenId") && // user must receive SigmaUSD
      userOut.tokens(0)._2 >= \$scAmountL && // the amount of SigmaUSD must be at least what user is paying for
      HEIGHT < \$timestampL // this part is always true (timestamp is the unix-timestamp at the time of the request), it will cause compiled address to differ everytime
  }
  
  // ensuring dApp integrity is preserved - any dApp specific condition to ensure designed procedures won't be violated
  val UIFeeOk = OUTPUTS(2).propositionBytes == fromBase64("\$implementor") && OUTPUTS.size == 4 // UI fee must go to UI devs not any random person who assembles the transaction
  val properBank = OUTPUTS(0).tokens(2)._1 == fromBase64("\$bankNFT") // the real bank box of the sigmaUSD protocol must be used so not any random person can behave as the bank box
  val dAppWorksFine = properFundUsage && UIFeeOk && properBank

  // in any case, whether assembler refuses to execute the request or the request fails for any reason, user must be able to get back his funds
  val returnFunds = { 
    val total = INPUTS.fold(0L, {(x:Long, b:Box) => x + b.value}) - \$returnFee // only refund transactions's fee must be deducted from user's funds
    OUTPUTS(0).value >= total && OUTPUTS(0).propositionBytes == fromBase64("\$userAddress") // user must receive the appropriate amount
    && (PK("\$assemblerNodeAddr") || HEIGHT > \$refundHeightThreshold) && // either dApp-specific node can return user's funds or some time (block) has to be passed first. This is useful for many reasons.
    OUTPUTS.size == 2 // only refund box and transaction fee box is needed
  }

  sigmaProp(dAppWorksFine || returnFunds) // either dApp must work as it is supposed to or user's funds must be returned
}