def dip  { swap >r call r> }
def keep { over >r call r> }
def if   { ? call }

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 ] call \=> 12586269025