@Merirosvo i think there is a more useful way of looking at things.
The mental construct of a 'constitution' has a social function. Sure societies can exist without it, but where a country is ruled by a constitution, it is living up to some function. By understanding this social function we can understand the essence of the constitution.
Again, i think the idea that a state derives its power from a set of written things (laws& precedent or a single document) is the mental construct. The function them is to define how power should be used; ie some limits on that power, and to describe some of the right of citizens.
Thus for example, in the US this might limit the power of the state to invade the privacy of the citizens. It might limit how the armed forces can be used within the country (to prevent a cue d'etat). Or many other things which might serve the state in some way.
The idea of a constitution is always to limit the abuse of power. Whether that is an absolute monarh giving up some power, or a newly formed republic trying to define some freedoms which they strive for.
What happened in Nepal was a huge mess, with various governing coalitions forming, Maoists and Leninist-Stalinists, and supporters of the old royal system all trying to aort out the cluster fuck and establish a federal system based on a constitution which they failed to write/agree on. Along with attempts to integrate the former maoist rebel forces into the standing armed forces, and disagreements over that...
You can't just say any tradition is a constitution. But it does remind me of a game of thrones quote/puzzle. 'A rich man, a king, and a priest each command a single soldier to kill the other two, who survives?', 'well the rich man promises money, the king commands loyalty, and the priest promises heaven in the afterlife, i guess it depends on what the soldier wants.' - 'exaclty, the truth is, power lies where men believe it lies'
(Ok, completely messing up that reference...) But the point is, if people believe that the constitution holds power then it does. If people believe in the Divine right of Kings then... Yadda yadda yadda.
The social function of a constitution is to have the rules written down; to make them clear so anyone can read them; thus allowing people challenge the actions of those who would ignore them - thus giving the constitution the power to have itself enforced.
If you have ever had a class contract, or similar group agreement, then you are benefitting from this idea.