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
  H : ∀ (n : nat) (x : A), P n x → f n x = x
  H0 : mapi_rec f tl (S n) = tl
  ============================
   (f n hd :: mapi_rec f tl (S n) = hd :: tl)


DEBUG:Going to execute:
constructor 1
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := f n hd :: mapi_rec f tl (S n) = hd :: tl
of type
constr of type tacvalue