problem with unary_compose

hi :)

i'm trying to use the "functional extensions" of the stl.
having a "problem" with the unary_compose..
The following code seems right to me, but doesnt compile -
if i declare "operator()" of "struct one" as const it works - but
i dont understand why it has to be static .. ?!

#include <functional>
#include <iostream>

#include <ext/functional>

using namespace std;
using namespace __gnu_cxx;

struct one: public unary_function<int, int>
{

int operator()(int arg)
{
return ++arg;
}

};

int main(int argc, char** argv) {

one o1,o2;
int i = 2;

unary_compose<one,onefunc = compose1(o1,o2);

cout<<func(i)<<endl;

return (EXIT_SUCCESS);
}