Goal:
    A : Type  B : Type  f : nat → A → B  l : list A  n : nat
  ============================
   (mapi_rec f [] (#|l| + n) =
    mapi_rec (λ (n : nat) (x : A), f (#|l| + n) x) [] n)


DEBUG:Going to execute:
fun => <coq-core.plugins.ltac::reflexivity@0>