// a namespace in C++ for usi/math_library

namespace usi {

int faculty_meeting();
int sum(int a, int b);

namespace math_library {

int sum(int a, int b);
int multiply(int a, int b);

}

}

using usi::faculty_meeting;
using usi::sum;
using namespace usi::math_library;

int main() {
    int i = usi::sum(10, 20);
    int j = sum(10, 20);	// Error: ambiguous!
    i += faculty_meeting();
}