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

def fac {
  dup if: 1 <= [drop 1] [dup 1 - fac *];
}

def fib {
  dup if: 1 <= [] [dup 1 - fib swap 2 - fib +];
}

[ 20 fib ] call \=> 6765
[ 10 fac ] call \=> 3628800