Goal:
  
  A : Type
  B : Type
  P : A → nat → B → Type
  a : B
  ls1 : list B
  n : nat
  X : All (λ x : A, Alli (P x) n (a :: ls1)) []
  ============================
   (All (λ x : A, Alli (P x) (S n) ls1) [])


DEBUG:Going to execute:
trivial; hnf; intros