[l_t]

Yep -- under the hood, "immutability" in Clojure is implemented with data structures that provide pretty good performance by sharing sections of their immutable structure with each other.

For example, if you have a vector of 100 items, and you "mutate" that by adding an item (actually creating a new vector), the language doesn't allocate a new 101-length vector. Instead, we can take advantage of the assumption of immutability to "share structure" between both vectors, and just allocate a new vector with two items (the new item, and a link to the old vector.) The same kind of idea can be used to share structure in associative data structures like hash-maps.

I'm no expert on this, so my explanation is pretty anemic and probably somewhat wrong. If you're curious, the book "Purely Functional Data Structures" [0] covers these concepts in concrete detail.

[0]: https://www.amazon.com/Purely-Functional-Data-Structures-Oka...