svoid pnlFieldValueComparison(O obj1, O obj2) {
  for (S field : joinTreeSets(allFields(obj1), allFields(obj2))) {
    O a = get(obj1, field);
    O b = get(obj2, field);
    if (a != null || b != null) {
      print(field + ": " +
        (a == b ? "same " + a
          : eq(a, b) ? "eq " + a
          : "diff " + a + " => " + b));
    }
  }
}