Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : A → B → bool
  f : C → A
  g : D → B
  a : C
  l : list C
  d : D
  l' : list D
  IHl : ∀ l' : list D,
          forallb2 R (map f l) (map g l') =
          forallb2 (λ (x : C) (y : D), R (f x) (g y)) l l'
  ============================
   (R (f a) (g d) && forallb2 R (map f l) (map g l') =
    R (f a) (g d) && forallb2 (λ (x : C) (y : D), R (f x) (g y)) l l')


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.