class Fib
    private function fib_i(integer n)
        return iff(n<2?n:this.fib_i(n-1)+this.fib_i(n-2))
    end function
    public function fib(integer n)
        if n<0 then throw("constraint error") end if
        return this.fib_i(n)
    end function
end class
Fib f = new()

function fib_i(integer i)
    return sprintf("this is not the fib_i(%d) you are looking for\n",i)
end function

?f.fib(10)
--?f.fib_i(10)  -- illegal
?fib_i(10)