Goal:
    A : Type  B : Type  f : nat → A → B  k : nat  l : list A
  ============================
   (skipn 0 (mapi_rec f l k) = mapi_rec f (skipn 0 l) (0 + k))


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