static <A extends BackgroundWithBoxes> A varyBackgroundWithBoxes(A s) {
  s = cloneThroughStructure(s);
  if (oneIn(10))
    s.background = varyColor(s.background);
  else if (tossCoin())
    replaceRandomElement(s.boxes, randomBox());
  else
    varyBox(random(s.boxes));
  ret s;
}