+coinductive break_spec (T : Type) (n : nat) (l : list T) : list T → CProp ≝
+| break_to: ∀l1,x,l2. \len l1 = n → l = l1 @ [x] @ l2 → break_spec T n l l.
+
+lemma list_break: ∀T,n,l. n < \len l → break_spec T n l l.
+intros 2; elim n;
+[1: elim l in H; [cases (not_le_Sn_O ? H)]
+ apply (break_to ?? ? [] a l1); reflexivity;
+|2: cases (H l); [2: apply lt_S_to_lt; assumption;] cases l2 in H3; intros;
+ [1: rewrite < H2 in H1; rewrite > H3 in H1; rewrite > append_nil in H1;
+ rewrite > len_append in H1; rewrite > plus_n_SO in H1;
+ cases (not_le_Sn_n ? H1);
+ |2: apply (break_to ?? ? (l1@[x]) t l3);
+ [2: simplify; rewrite > associative_append; assumption;
+ |1: rewrite < H2; rewrite > len_append; rewrite > plus_n_SO; reflexivity]]]
+qed.
+
+include "logic/cprop_connectives.ma".