code: func countSpentOutputs(block *btcutil.Block) int {
  	// Exclude the coinbase transaction since it can't spend anything.
  	var numSpent int
  	for _, tx := range block.Transactions()[1:] {
  		numSpent += len(tx.MsgTx().TxIn)
  	}
  	return numSpent
  }
docs: // countSpentOutputs returns the number of utxos the passed block spends.