Hint Extern 1 (lc_co (g_AbsCong _ _ _)) =>
  let x1 := fresh in
  pick_fresh x1;
  apply (lc_g_AbsCong_exists x1).Hint Extern 1 (lc_co (g_CAbsCong _ _ _)) =>
  let c1 := fresh in
  pick_fresh c1;
  apply (lc_g_CAbsCong_exists c1).

Lemma co_subst_co_co_open_co_wrt_tm : forall g2 g1 a1 c1, lc_co g1 -> co_subst_co_co g1 c1 (open_co_wrt_tm g2 a1) = open_co_wrt_tm (co_subst_co_co g1 c1 g2) (co_subst_co_tm g1 c1 a1).