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) × Alli P (#|a :: l| + n) l')


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

In environment
A : Type
P : nat → A → Type
n : nat
l' : list A
H : Alli P n ([] ++ l')
Unable to unify "Alli ?M2501 ?M2502 (?M2503 :: ?M2504)" with
 "Alli P (#|[]| + n) l'".
Level 0: Backtrace:

In environment
A : Type
P : nat → A → Type
n : nat
l' : list A
H : Alli P n ([] ++ l')
Unable to unify "Alli ?M2501 ?M2502 (?M2503 :: ?M2504)" with
 "Alli P (#|[]| + n) l'".
DEBUG LTAC Evaluated term: H