Method move_to

code/geometry/rotating_calipers.cpp:15–15

    angle -= by;
    while (angle < 0) angle += 2*pi; }
  void move_to(ii pt2) { pt = pt2; }
  double dist(const caliper &other) {
    point a(pt.first,pt.second),
       b = a + exp(point(0,angle)) * 10.0,