#ifndef COMPOSER_FGX1_GX2
#define COMPOSER_FGX1_GX2
#include <functional>   //for binary_function
using namespace std;

//Compose the functions f(g(x1), g(x2)).

template <class F, class G>
class composer_fgx1_gx2: public binary_function<typename G::argument_type,
                                                typename G::argument_type,
                                                typename F::result_type> {
	F f;
	G g;
public:
	composer_fgx1_gx2(const F& initial_f, const G& initial_g)
		: f(initial_f), g(initial_g) {}

	typename F::result_type operator()(
		const typename G::argument_type& x1,
		const typename G::argument_type& x2) {
		return f(g(x1), g(x2));
	}
};

template <class F, class G>
inline composer_fgx1_gx2<F, G> compose_fgx1_gx2(
	const F& initial_f,
	const G& initial_g)
{
	return composer_fgx1_gx2<F, G>(initial_f, initial_g);
}
#endif