Goal:
  
  A : Type
  P : A → A → Type
  Q : A → A → Type
  x : A
  y : A
  l : list A
  l' : list A
  X : All2 (Trel_conj P Q) l l'
  IHX : All2 P l l' × All2 Q l l'
  a : P x y
  b : Q x y
  ============================
   (All2 P l l')


DEBUG:Going to execute:
repeat
            match goal with
            | |- ?X1 → ?X2 => intro
            | |- ¬ _ => unfold not at 1; intro
            end