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
  pa : is_true false
  pl : is_true (alli p (S n) l)
  f : P n a → False
  ============================
   (P n a)


DEBUG:Going to execute:
reflexivity
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.