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


DEBUG:Going to execute:
DepElim.depelim H
Level 3: evaluation returns
DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim;
 CoreTactics.unblock_goal
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := Hl
       of type
       constr
of type
tacvalue
H := Hl
of type
constr of type tacvalue

Level 2: evaluation returns
DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim;
 CoreTactics.unblock_goal
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := Hl
       of type
       constr
of type
tacvalue
H := Hl
of type
constr of type tacvalue

Level 1: evaluation returns
DepElim.do_depelim_nosimpl tac H; DepElim.simpl_dep_elim;
 CoreTactics.unblock_goal
where
tac := fun hyp => DepElim.do_case hyp
       where
       id := Hl
       of type
       constr
of type
tacvalue
H := Hl
of type
constr of type tacvalue