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


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"