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
  z1 : sigma (λ _ : list A, list B)
  z : All2 (λ (x : A) (y : B), R x (g x y)) (pr1 z1) (pr2 z1)
  ============================
   ({| pr1 := x :: xs; pr2 := y :: ys |} = z1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2 R xs (map2 g xs ys))


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 := x :: xs; pr2 := y :: ys |}
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