Goal:
    x : byte  y : byte  Hcomp : Some x = Some y
  ============================
   (let H := CoreTactics.block in x = y)


DEBUG:Going to execute:
CoreTactics.on_last_hyp
            ltac:(fun id' =>
                    match id' with
                    | id => idtac
                    | _ => revert id'; CoreTactics.revert_until id
                    end)