Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl : #|[]| = #|[]|
  ============================
   (All2 Q [] [])


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