Goal:
    A : Type  B : Type  f : nat → A → B  l : list A
  ============================
   (mapi_rec f (List.rev l) 0 =
    List.rev (mapi (λ (k : nat) (x : A), f (#|l| - S k) x) l))


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