Just finished chapter 13, which added flow control. Recursive functions are now possible like one that calculates the Fibonacci sequence. It is not that fast, even compared to Python but at least it works.

lzp> def {fib} (\ {n} {if (<= n 1) {n} {+ (fib (- n 1)) (fib (- n 2))}})
()
lzp> fib 25
75025