Goal:
    A : Type  B : Type  C : Type  R : A → B → C → Type  H : True
  ============================
   (All3_nil = All3_nil)


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