Goal:
    A : Type  P : A → Type  a : A  X : All P [a]
  ============================
   (let H := CoreTactics.block in P a)


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