Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  Q : A → B → Prop
  x : A
  y : B
  l : list A
  l' : list B
  H : P x y
  H0 : Forall2 P l l'
  IHForall2 : Forall (λ x : A, ∀ y : B, P x y → Q x y) l → Forall2 Q l l'
  H1 : Forall (λ x : A, ∀ y : B, P x y → Q x y) (x :: l)
  x0 : A
  l0 : list A
  H4 : ∀ y : B, P x y → Q x y
  H5 : Forall (λ x : A, ∀ y : B, P x y → Q x y) l
  H2 : x0 = x
  H3 : l0 = l
  ============================
   (Forall2 Q l l')


DEBUG:Going to execute:
F x2 x1 x0
Level 0: Backtrace:

P is used in hypothesis H.
Level 0: Backtrace:

P is used in hypothesis H.
Level 0: Backtrace:

P is used in hypothesis H.
Level 0: Backtrace:

P is used in hypothesis H.
Level 0: Backtrace:

P is used in hypothesis H.
Level 1: Backtrace:

P is used in hypothesis H.
Level 0: Backtrace:

P is used in hypothesis H.