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
  Hl1 : nat
  Hl : S #|l'| = Hl1
  ============================
   (0 = Hl1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2 Q [] (b :: l'))


DEBUG:Going to execute:
try rename id into id',id' into id
Level 3: evaluation returns
try rename id into id',id' into id; cont id'1 id
where
t := 0
of type
constr
id'1 := Hl1
of type
constr
id' := Hl
of type
constr
id := Hl
of type
constr
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := Hl
        of type
        constr
of type
tacvalue of type tacvalue