Goal:
  
  A : Type
  P : nat → A → Type
  f : nat → A → A
  n : nat
  hd : A
  tl : list A
  p : P n hd
  X : Alli P (S n) tl
  IHX : (∀ (n : nat) (x : A), P n x → f n x = x) → mapi_rec f tl (S n) = tl
  H : ∀ (n : nat) (x : A), P n x → f n x = x
  ============================
   (f n hd :: mapi_rec f tl (S n) = hd :: tl)


DEBUG:Going to execute:
F x2 x1 x0
Level 2: evaluation returns
<coq-core.plugins.tauto::find_cut@0>
where
k := fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]
     where
     flags := 
     of type
     tauto_flags
of type
tacvalue of type tacvalue