// rotation: rotation in degrees (clockwise)
static PrecisePolygon regularPolygon(IDoublePt etc center, double radius, int sides, double rotation default 0) {
  double angle = twoPi()/sides;
  reMutable rotation = degToRadian(-rotation);
  ret new PrecisePolygon(countIteratorToList(sides, i ->
    addDoublePts(center, scaleDoublePt(radius,
      doublePt(sin(rotation+angle*i), cos(rotation+angle*i))))));
}