Goal:
  
  A : Type
  P : nat → A → Type
  p : nat → A → bool
  n : nat
  a : A
  l : list A
  Hp : ∀ (i : nat) (x : A), reflectT (P i x) (p i x)
  IHl : ∀ n : nat, alli p n l → Alli P n l
  H : is_true (p n a && alli p (S n) l)
  ============================
   (Alli P n (a :: l))


DEBUG:Going to execute:
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 ]