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
  ============================
   (mapi_rec f (List.rev (l ++ [x])) n =
    List.rev
      (mapi_rec (λ (k : nat) (x0 : A), f (#|l ++ [x]| + n - S k) x0)
         (l ++ [x]) 0))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

Tactic failure: congruence failed.