Not sure how many constitutions around the world have a "Secession Procedure" section, but I suppose not too many. I know UK sort of has one in its "unwritten" constitution. I understand that Spanish government cannot simply ignore the constitution and must do something, I'm sure they could have come up with a better something...

I expect in vast majority of cases of gaining independence from a country with existing constitution it wasn't something along the lines of: "OK, so you want independence? Fine, just wait a bit until we change the constitution". Then again, and sadly, it probably wasn't happening without violence either.

Here's a list of unilateral declarations of independence, some more successful than others:

https://en.wikipedia.org/wiki/Unilateral_declaration_of_inde...

And let's not forget that a lot of cases of gaining independence happened after someone lost a war. Maybe let's not go there?