Closures Data Structures Are Trees of Arrays, Right?

Closures data structures are trees of arrays. They have a very high branching factor and they're very shallow. And that ends up being very fast. So I guess that's the whole thing about immutability, which again, if you don't share anything that is mutable, you don't have to lock. But then of course, people will say that if you never change anything, how sensible can a program be? so I guess at some point you need to be able to change some things.