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 : S #|l'| = 0
  Hl0 := {| pr1 := 0; pr2 := Hl |} : sigma (λ index : nat, S #|l'| = index)
  Hl1 := 0 : nat
  Hl2 := Hl : (λ index : nat, S #|l'| = index) Hl1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All2 Q [] (b :: l'))


DEBUG:Going to execute:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2