Babel Fee Box Pattern (Pay Fees in Any Token)

{
  // Babel Fee Box: offers tokens for ERG
  // R4: Token ID being offered
  // R5: Token amount offered
  // R6: ERG amount requested
  // R7: User's address (for any leftover)
  
  val offeredTokenId = SELF.R4[Coll[Byte]].get
  val offeredAmount = SELF.R5[Long].get
  val requestedErg = SELF.R6[Long].get
  val userAddress = SELF.R7[Coll[Byte]].get
  
  // Verify box has the offered tokens
  val hasTokens = SELF.tokens(0)._1 == offeredTokenId &&
                  SELF.tokens(0)._2 >= offeredAmount
  
  // Babel box can be consumed if:
  // 1. User receives the requested ERG, OR
  // 2. User cancels (returns tokens to self)
  
  val fulfilled = {
    // User receives ERG
    OUTPUTS.exists(o =>
      o.propositionBytes == userAddress &&
      o.value >= requestedErg
    )
  }
  
  val cancelled = {
    // User gets tokens back
    OUTPUTS.exists(o =>
      o.propositionBytes == userAddress &&
      o.tokens(0)._1 == offeredTokenId &&
      o.tokens(0)._2 >= offeredAmount
    )
  }
  
  hasTokens && (fulfilled || cancelled)
}