static void drawArrowBetweenRects(Graphics2D g, Rect r1, Rect r2, Color color) {
  // TODO: more cases
  Pt p1, p2;
  if (r2.x > r1.x2()) {
    p1 = pt(r1.x2(), middleOfY(r1));
    p2 = pt(r2.x-1, middleOfY(r2));
  } else {
    p1 = pt(middleOfX(r1), r1.y-1);
    p2 = pt(middleOfX(r2), r2.y+r2.h);
  }
  drawArrowBetweenPoints(g, p1, p2, color);
}