Goal:
  
  A : Type
  B : Type
  f : nat → B
  a : A
  l : list A
  IHl : ∀ f : nat → B,
          mapi_rec (λ (i : nat) (_ : A), f i) l 0 = unfold #|l| f
  ============================
   (f 0 :: mapi_rec (λ (n : nat) (_ : A), f (S n)) l 0 =
    unfold #|l| f ++ [f #|l|])


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