Goal:
  
  A : Type
  a : A
  l : list A
  i : nat
  a0 : A
  IHl : skipn i l = a0 :: skipn (S i) l
  ============================
   (skipn i l = a0 :: skipn (S i) l)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
DEBUG LTAC Evaluated term: (IHl i)
DEBUG LTAC Evaluated term: nth_error