Goal:
  
  A : Type
  v : A
  _a_ : A
  _l_ : list A
  n : nat
  H : hd_error (skipn n _l_) = Some v
  ============================
   (hd_error (skipn (S n) (_a_ :: _l_)) = Some v)


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