number area2(const G& P) { number A = 0; for (int i = 0; i < P.size(); ++i) A += cross(curr(P, i), next(P, i)); return A; }
/* polygon area * 2 */ double area2(const G& P) { double A = 0; for (int i = 0; i < P.size(); ++i) A += cross(curr(P, i), next(P, i)); return A; }