Fixpoint linsert (t1 : tuple) (l : line) :=
match l with
|   nil => t1 :: l
| t2::l1 => match tcompare t1 t2 with
|  Gt => t2 :: linsert t1 l1
| res => t1 :: l
end
end.

Lemma ltsort_c c l :
let (s,l1) := ltsort c l in
(s2k s * {l[l1]})%f = (s2k c * {l[l]})%f.