Goal:
  
  A : Type
  B : Type
  P : A → Prop
  p : A → bool
  f : A → B
  g : A → B
  x : A
  l : list A
  H : P x
  H0 : Forall P l
  IHForall : forallb p l
             → (∀ x : A, P x → p x → f x = g x) → map f l = map g l
  px : is_true (p x)
  pl : is_true (forallb p l)
  Hx : ∀ x : A, P x → p x → f x = g x
  ============================
   (is_true (p x))


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"