Goal:
  
  A : Type
  P : nat → A → Type
  n : nat
  a : A
  l : list A
  l' : list A
  IHl : ∀ (n : nat) (l' : list A),
          Alli P n (l ++ l') → Alli P n l × Alli P (#|l| + n) l'
  X : P n a
  X0 : Alli P (S n) (l ++ l')
  ============================
   (Alli P n (a :: l))


DEBUG:Going to execute:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end