(** * Properties of sub *)

Lemma sub_sub (s : string) (off1 len1 off2 len2 : int) :
to_Z off1 + to_Z off2 = to_Z (off1 + off2)%uint63 ->
to_Z len2 <= to_Z len1 - to_Z off2 ->
sub (sub s off1 len1) off2 len2 = sub s (off1 + off2)%uint63 len2.