Goal:
    A : Type  P : list A → A → Type  x : A  a : All_fold P [x]
  ============================
   (P [] x)


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