proposition: Lemma gorgeous_plus13 n: gorgeous n -> gorgeous (n + 13).
proof: Proof. move=> H. apply: (g_plus5 _ (n + 8))=>//; last by rewrite -addnA; congr (_ + _). apply: (g_plus5 _ (n + 3)); last by rewrite -addnA; congr (_ + _). by apply: (g_plus3 _ n). Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq)