Goal:
    z : t  hd : A.t  tl : list A.t  H : lt (hd :: tl) z
  ============================
   (let H := CoreTactics.block in lt [] z)


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