;; (FIB) generates the Fibonacci sequence and emits sequential (index . element) pairs.
!(set-env (letrec ((next (lambda (a b n)
                            (next b
                                  ;; We are using IF as a poor-man's PROGN.
                                  ;; The condition is evaluated first, then one of the branches is evaluated.
                                  (+ (if (emit (cons n a)) a a)
                                     b)
                                  (+ 1 n))))
                    (fib (lambda () (next 0 1 0))))
             (current-env)))