The Book of Jashar is also mentioned in Joshua 10:13. It is understood to be a Scroll of the Righteous
that deals with the heroic exploits of the Israelites.1 The fact that this lament is found in this book suggests that it has something to do with preparing Israelites for (future) acts of heroism.
18 et qu'il ordonna d'enseigner aux enfants de Juda. C'est le cantique de l'arc; il est écrit dans le livre du Juste.