Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  p : A → B → bool
  Hp : ∀ (x : A) (y : B), reflect (P x y) (p x y)
  x : A
  y : B
  l : list A
  l' : list B
  H : P x y
  H0 : Forall2 P l l'
  IHForall2 : is_true (forallb2 p l l')
  ============================
   (is_true (p x y && forallb2 p l l'))


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | Ltac.split ]
            | case not_locked_false_eq_true; Ltac.assumption
            | match goal with
              | H:¬ _ |- _ => solve [ case H; trivial ]
              end ])