Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  a : A
  l : list A
  a0 : A
  l' : list A
  IHl : All P l → All Q l' → #|l| = #|l'| → All2 (λ x y : A, P x ∧ Q y) l l'
  p : P a
  ha : All P l
  hb : All Q (a0 :: l')
  hb0 := {| pr1 := a0 :: l'; pr2 := hb |}
      : sigma (λ index : list A, All Q index)
  hb1 := a0 :: l' : list A
  hb2 := hb : (λ index : list A, All Q index) hb1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    S #|l| = S #|l'| → All2 (λ x y : A, P x ∧ Q y) (a :: l) (a0 :: 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