static S cal_structure(CirclesAndLines cal) {
  ret cal_structure_impl(restructure(cal));
}

static S cal_structure_impl(CirclesAndLines cal) {
  if (cal.arrowClass == Arrow) cal.arrowClass = null;
  if (cal.circleClass == Circle) cal.circleClass = null;
  for (Circle c : cal.circles) cal_structure_simplifyElement(c);
  for (Line l : cal.lines) cal_structure_simplifyElement(l);
  ret structure(cal);
}

static void cal_structure_simplifyElement(Base b) {
  if (eq(b.traits, ll(b.text))) b.traits = null;
}