Each step of a lazy seq starts as a closure that knows how to compute a cons
cell (first and rest). When you ask for either, that closure is called and its
return value is cached. Next time that first or test is called, the cached
value is returned.

So as long as you refer only to parts of the seq that have already been realized
and cached, nothing has to recurse at that point.

And that’s what happens with ‘fibs’ — each element is computed only from
earlier elements — all the way back to the [0 1] constants.

(def fibs (lazy-cat [0 1] (map + fibs (rest fibs))))

Is that def just refering to itself?! How does that work?