Consensus Engine separation

Validation of headers

Validation of uncles (EtHash)

Use of smart contract state for Consensus Engine

Solution for Staged sync

Fork choice rule

For choice rule can be thought of a partial order relationship among the set of possible headers. Being partial order, fork choice rule is:

1. reflexive or irreflexive, depending on whether non-strict or strict definition is required. header A is either better than itself (non-strict, <=), or not better than itself (strict, <)
2. anti-symmetric. if A better than B, then B is worse than A)
3. transitive. if A better than B and B is better than `

Core is asking the Consensus Engine to infer the relation between given headers, and perform topological sort.