Finished fairness protocol section

contrib/fairness.asciidoc

@@ -58,3 +58,17 @@ This is only a small list of a whole "zoo" of security and cryptographic primiti
 In our real-life example, we saw one form of fairness protocol called "split and choose". This is just one of a myriad different fairness protocols that can be built by combining the building blocks of security primitives in different ways. But the basic pattern is always the same: two or more participants interact without trusting each other, by engaging in a series of steps that are part of an agreed protocol. The protocol's steps arrange incentives and disincentives to ensure that if the participants are rational, cheating is counter-productive and fairness is the automatic outcome. Enforcement is not necessary to get fair outcomes - it is only necessary to keep the participants from breaking out of the agreed protocol.
 
 Now that you understand this basic pattern, you will start seeing it everywhere in Bitcoin, the Lightning Network and many other systems. Let's look at some specific examples, next.
+
+==== Example of the fairness protocol
+
+The most prominent example of a "fairness protocol", is Bitcoin's consensus algorithm _Proof of Work_ (PoW). In Bitcoin, miners compete to verify transactions and aggregate them in blocks. To ensure that the miners do not cheat, without entrusting them with authority, Bitcoin uses a system of incentives and disincentives. Miners have to use a lot of electricity doing "work", that is embedded as a "proof" inside every block. This is achieved because of a property of hash functions where the output value is randomly distributed across the entire range of possible outputs. If miners succeed in producing a valid block fast enough, they are rewarded by earning the block reward for that block.  Forcing miners to use a lot of electricity before the network considers their blocks means that they have an incentive to correctly validate the transactions in the block. If they cheat or make any kind of mistake, their block is rejected and the electricity they used to "prove" it is wasted. No one needs to force miners to produce valid blocks, the reward and punishment incentivize them to do so. All the protocol needs to do is ensure that only valid blocks with proof of work are accepted.
+
+The "fairness protocol" pattern can also be found in many different aspects of the Lightning Network:
+
+* Those who fund channels make sure that they have a refund transaction signed before they publish the funding transaction.
+
+* Whenever a channel is moved to a new state, the old state is "revoked" by ensuring that if anyone tries to broadcast it, they lose the entire balance and get punished.
+
+* Those who forward payments know that if they commit funds forward, they can either get a refund or they get paid by the node preceding them.
+
+Again and again, we see this pattern. Fair outcomes are not enforce by any authority. They emerge as the natural consequence of a protocol that rewards fairness and punishes cheating. A fairness protocol that harnesses self-interest by directing it towards fair outcomes.