Introducing the "minimal" CBC Casper Consensus Protocols.

CBC Casper is a family of "correct-by-construction" consensus protocols that share the same proof of asynchronous, Byzantine fault tolerant consensus safety. We describe this family of protocols by defining their protocol states and protocol state transitions, and then we provide a proof of Byzantine fault tolerant consensus safety for the entire Minimal CBC Casper family of protocols. We give examples of members of this family of protocols, including a binary consensus protocol. "Casper the Friendly Ghost", a blockchain consensus protocol, and a sharded blockchain consensus protocol. Each of these examples is "correct-by-construction" because the way they are defined guarantees that they are part of this family of protocols, and therefore satisfy the consensus safety theorem. This draft is intended to be introductory and educational, it is not a complete description of a system that can be implemented exactly as specified.