Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  f : A → B
  g : B → C → D
  a : A
  l : list A
  c : C
  l' : list C
  IHl : ∀ l' : list C,
          map2 g (map f l) l' = map2 (λ (x : A) (y : C), g (f x) y) l l'
  ============================
   (map2 g (map f l) l' = map2 (λ (x : A) (y : C), g (f x) y) l l')


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