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


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