Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  x : A
  l : list A
  p : P x
  H1 : All P l
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl : #|[]| = #|x :: l|
  IHAll : ∀ l' : list B,
            (∀ (x : A) (y : B), In y l' → P x → Q x y)
            → #|l'| = #|l| → All2 Q l l'
  ============================
   (All2 Q (x :: l) [])


DEBUG:Going to execute:
match H' with
           | H => idtac
           end
Level 3: H'
 has value 
IHAll

Level 3: Backtrace:

No matching clauses for match.
Level 2: Backtrace:

No matching clauses for match.