void cut(points &p, point b, point a, points &res) {
res.clear();
int n = p.size();
for (int i = 0; i < n; i ++) {
point c = p[i];
point d = p[next(i)];
if (dcmp(det(b - a, c - a)) >= 0) res.push_back(c);
if (dcmp(det(b - a, c - d)) != 0) {
point cp = interpoint(line(a, b), line(c, d));
if (onseg(cp, c, d)) res.push_back(cp);
}
}
}
