def fib/aux {
  if: dig dup 0 =
    [drop drop]
    [bury [swap 1 - swap] dip dup [+] dip swap fib/aux];
}
def fib { 0 1 fib/aux }

"50 fib => " type
50 fib .

def times {
  if: over 0 =
    [drop drop]
    [swap over >r >r call r> 1 - r> times];
}

\ We can also calculate the Fibonnaci numbers using the `times' combinator
\ we just implemented:

def fib-iter {
  0 1 dig times: [dup [+] dip swap]; drop
}

"50 fib-iter => " type
50 fib-iter .