Function parallel

code/geometry/lines.cpp:4–5

bool collinear(L(a, b), L(p, q)) {
  return abs(ccw(a, b, p)) < EPS && abs(ccw(a, b, q)) < EPS; }
bool parallel(L(a, b), L(p, q)) {
  return abs(cross(b - a, q - p)) < EPS; }
point closest_point(L(a, b), P(c), bool segment = false) {
  if (segment) {
    if (dot(b - a, c - b) > 0) return b;