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')
  x0 : A
  y : B
  l : list A
  l' : list B
  r : R x0 (g x0 y)
  z : All2 (λ (x : A) (y : B), R x (g x y)) l l'
  ============================
   ({| pr1 := x :: xs; pr2 := [] |} = {| pr1 := x0 :: l; pr2 := y :: l' |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2 R (x :: xs) [])


DEBUG:Going to execute:
simplify * || simplify ?; cbv beta