Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → C → Type
  g : A → B → C
  x : A
  xs : list A
  ih : ∀ l' : list B,
         All2 (λ (x : A) (y : B), R x (g x y)) xs l'
         → All2 R xs (map2 g xs l')
  y : B
  ys : list B
  z : All2 (λ (x : A) (y : B), R x (g x y)) (x :: xs) (y :: ys)
  z0 := {| pr1 := {| pr1 := x :: xs; pr2 := y :: ys |}; pr2 := z |}
     : sigma
         (λ index : sigma (λ _ : list A, list B),
            All2 (λ (x : A) (y : B), R x (g x y)) (pr1 index) (pr2 index))
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in R x (g x y))


DEBUG:Going to execute:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
            hnf in (value of id'1), (value of id'2)