Goal:
  
  A : Type
  B : Type
  C : Type
  R : nat → A → C → Type
  g : A → B → C
  x : A
  xs : list A
  ih : ∀ (l' : list B) (k : nat),
         All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k xs l'
         → All2i R k xs (map2 g xs l')
  k : nat
  z1 : sigma (λ _ : nat, sigma (λ _ : list A, list B))
  z : All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) 
        (pr1 z1) (pr1 (pr2 z1)) (pr2 (pr2 z1))
  ============================
   ({| pr1 := k; pr2 := {| pr1 := x :: xs; pr2 := [] |} |} = z1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2i R k (x :: xs) [])


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 := {| pr1 := k; pr2 := {| pr1 := x :: xs; pr2 := [] |} |}
of type
constr
id'1 := z1
of type
constr
id' := z
of type
intropattern
id := z
of type
intropattern
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := z
        of type
        intropattern
of type
tacvalue of type tacvalue