sparse sparse :: operator+ (const sparse& s) {
    sparse ans(this*);
    int sn = sizeof(s.xy) / (2 * sizeof(int));
    for (int i = 0; i < sn; ++i)
        ans[s.xy[i]] += s.A[i];

    int n = sizeof(ans.xy) / (2 * sizeof(int)), k = 0;
    for (int i = 0; i < n; ++i) {
        if (ans.A[i] == 0) {
            ++k;
            ans.xy[i] = ans.xy[n - 1 - k];
            ans.A[i] = ans.A[n - 1 - k];
        }
    }

    indices new_xy = new indices[n - k];
    double new_A = new double[n - k];
    for (int i = 0; i < n - k; ++i) {
        new_xy[i] = ans.xy[i];
        new_A[i] = ans.A[i];
    }
    delete[] ans.xy;
    delete[] ans.A;
    ans.xy = new_xy;
    ans.A = new_A;

    return ans;
}