Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : C → D → Type
  f : A → C
  g : B → D
  a : A
  l : list A
  b : B
  l' : list B
  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) (g b :: map g l')
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2 (λ (x : A) (y : B), R (f x) (g y)) (a :: l) (b :: l'))


DEBUG:Going to execute:
progress (autounfold with equations in e; simpl in e)
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.