Method operator +

code/mathematics/berlekamp_massey.test.cpp:9–11

        x = (_x % 1000000007 + 1000000007) % 1000000007;
    }
    Mod operator +(const Mod &other) const {
        return x + other.x;
    }
    Mod operator -(const Mod &other) const {
        return x - other.x;
    }