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:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"