Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : C → D → Type
  f : A → C
  g : B → D
  a : A
  l : list A
  IHl : ∀ l' : list B,
          All2 R (map f l) (map g l')
          → All2 (λ (x : A) (y : B), R (f x) (g y)) l l'
  H : All2 R (f a :: map f l) []
  H0 := {| pr1 := {| pr1 := f a :: map f l; pr2 := [] |}; pr2 := H |}
     : sigma
         (λ index : sigma (λ _ : list C, list D),
            All2 R (pr1 index) (pr2 index))
  H1 := {| pr1 := f a :: map f l; pr2 := [] |} : sigma (λ _ : list C, list D)
  H2 := H
     : (λ index : sigma (λ _ : list C, list D),
          All2 R (pr1 index) (pr2 index)) H1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2 (λ (x : A) (y : B), R (f x) (g y)) (a :: 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; hnf in id'2; 
            simpl in id'2; rename id'2 into id; cont id id id'1 t