Func _BigNum_gcf($a, $b)
    ;iterative (guessed)
    Local $sub
    If _BigNum_Compare($a,$b)<0 Then Return _BigNum_gcf($b,$a)

    While _BigNum_Compare($b,0)<>0;$b <> 0
        Local $c=_BigNum_Mod($a,$b)
        $a = $b
        $b = $c
    WEnd
    Return $a
EndFunc   ;==>_BigNum_gcf