Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  Q : A → B → Type
  l : list A
  l' : list B
  ============================
   (All2 P l l' → All2 Q l l' → All2 (Trel_conj P Q) l l')


DEBUG:Going to execute:
induction 1; intros H; depelim H; constructor