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


DEBUG:Going to execute:
fun tac H =>
             DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim;
              CoreTactics.unblock_goal