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 (λ (x : A) (y : C), P x (f y)) l l')


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]