Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → Prop
  f : C → B
  l : list A
  l' : list C
  H : Forall2 (λ (x : A) (y : C), P x (f y)) l l'
  ============================
   (Forall2 P (map (λ x : A, x) l) (map f l'))


DEBUG:Going to execute:
eapply Forall2_map
DEBUG LTAC Evaluated term: (map_id l)
DEBUG LTAC Evaluated term: (map_id l)