Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  x : A
  l : list A
  IHl : ∀ n : nat,
          mapi_rec f (List.rev l) n =
          List.rev (mapi_rec (λ (k : nat) (x : A), f (#|l| + n - S k) x) l 0)
  n : nat
  ============================
   (f n x
    :: List.rev (mapi_rec (λ (k : nat) (x : A), f (#|l| + S n - S k) x) l 0) =
    f (#|l ++ [x]| + n - S (#|l| + 0)) x
    :: List.rev
         (mapi_rec (λ (k : nat) (x0 : A), f (#|l ++ [x]| + n - S k) x0) l 0))


DEBUG:Going to execute:
PTRDEBUGTACfun => <coq-core.plugins.cc::f_equal@0>

File "./theories/MCList.v", line 510, characters 10-20:
Warning: Notation app_length is deprecated since 8.20.
Use length_app instead.
[deprecated-syntactic-definition-since-8.20,deprecated-since-8.20,deprecated-syntactic-definition,deprecated,default]