Orphan instances

The BlockSupportsDiffusionPipelining instance for the HFC is compositional:

• TentativeHeaderState (HardForkBlock xs) is OneEraTentativeHeaderState, the n-ary sum of per-era TentativeHeaderStates. The initialTentativeHeaderState is the one for the first era.
• Similarly, TentativeHeaderView (HardForkBlock xs) is OneEraTentativeHeaderView, the n-ary sum of per-era TentativeHeaderViews, and tentativeHeaderView delegates to the era-specific tentativeHeaderView.

• applyTentativeHeaderView Proxy thv st works like this:

  • If thv and st are in the same era, we call the corresponding era-specific applyTentativeHeaderView.
  • If thv is in a later era than st, we replace st with the initialTentativeHeaderState for the era of thv, and then proceed like in the previous step.
  • If thv is in an earlier era than st, we return Nothing to disallow pipelining.

This behavior guarantees the "Consistent validity under subsequences" requirement if it is satisfied for every era.

Note that at an era boundary, the tip of the selection might switch multiple times between two adjacent eras. Compared to the scenario where the pipelining criteria in both eras are compatible and make sense even across eras, this might lead to unnecessarily strict/relaxed diffusion pipelining. However, the tip switching between different eras is rare and rather short, so there is no direct need to address this, so we rather avoid the extra complexity for now.

Still, a possible future refinement would be to allow custom logic for "upgrading" the TentativeHeaderState to a new era.