ch06: add new image (UTXO + STXO + UTXO set)

ch06.asciidoc

@@ -64,7 +64,13 @@ You may also notice a lot of strange and indecipherable fields and hexadecimal s
 
 ((("transactions", "outputs and inputs", id="Tout06")))((("outputs and inputs", "outputs defined")))((("unspent transaction outputs (UTXO)")))((("UTXO sets")))((("transactions", "outputs and inputs", "output characteristics")))((("outputs and inputs", "output characteristics")))The fundamental building block of a bitcoin transaction is a  _transaction output_. Transaction outputs are indivisible chunks of bitcoin currency, recorded on the blockchain, and recognized as valid by the entire network. Bitcoin full nodes track all available and spendable outputs, known as _unspent transaction outputs_, or _UTXO_. The collection of all UTXO is known as the _UTXO set_ and currently numbers in the millions of UTXO. The UTXO set grows as new UTXO is created and shrinks when UTXO is consumed. Every transaction represents a change (state transition) in the UTXO set.
 
-((("balances")))When we say that a user's wallet has "received" bitcoin, what we mean is that the wallet has detected an UTXO that can be spent with one of the keys controlled by that wallet. Thus, a user's bitcoin "balance" is the sum of all UTXO that user's wallet can spend and which may be scattered among hundreds of transactions and hundreds of blocks. The concept of a balance is created by the wallet application. The wallet calculates the user's balance by scanning the blockchain and aggregating the value of any UTXO the wallet can spend with the keys it controls. Most wallets maintain a database or use a database service to store a quick reference set of all the UTXO they can spend with the keys they control.
+((("balances")))When we say that a user's wallet has "received" bitcoin, what we mean is that the wallet has detected on the blockchain an UTXO that can be spent with one of the keys controlled by that wallet. Thus, a user's bitcoin "balance" is the sum of all UTXO that user's wallet can spend and which may be scattered among hundreds of transactions and hundreds of blocks. The concept of a balance is created by the wallet application. The wallet calculates the user's balance by scanning the blockchain and aggregating the value of any UTXO the wallet can spend with the keys it controls. Most wallets maintain a database or use a database service to store a quick reference set of all the UTXO they can spend with the keys they control.
+
+((("spent transaction outputs (STXO)")))<<utxo-stxo>> displays the blockchain in three different moments, as the transaction chain from Joe to Gopesh is being built. Notice how each transaction spends an UTXO that was created in a previous transaction, turning it into a _spent transaction output_, or _STXO_. Since transaction #1 (from Joe to Alice) spends a single UTXO (from Joe) and creates a single UTXO (to Alice), it doesn't modify the UTXO set size. On the other hand, transactions #2 and #3 both create change outputs to the sender, spending a single UTXO and creating two UTXO (the payment and the change output). Therefore, each of them increases the UTXO set size by 1.
+
+[[utxo-stxo]]
+.Transaction chain from Joe to Gopesh being built on the blockchain
+image::images/mbc2_0609.png["Transaction chain from Joe to Gopesh being built on the blockchain"]
 
 ((("satoshis")))A transaction output can have an arbitrary (integer) value denominated as a multiple of satoshis.  Just as dollars can be divided down to two decimal places as cents, bitcoin can be divided down to eight decimal places as satoshis. Although an output can have any arbitrary value, once created it is indivisible. This is an important characteristic of outputs that needs to be emphasized: outputs are _discrete_ and _indivisible_ units of value, denominated in integer satoshis. An unspent output can only be consumed in its entirety by a transaction.