sS nicelyFormatJsonDiff(O diff) {
  new StringBuilderWithIndent buf;
  nicelyFormatJsonDiff(diff, buf);
  ret str(buf);
}

svoid nicelyFormatJsonDiff(O diff, StringBuilderWithIndent buf) {
  if (diff == null) ret with buf.println("No differences");
  if (diff cast JsonDiff.Removed)
    ret with buf.println("DELETE " + diff.a);
  if (diff cast JsonDiff.Added)
    ret with buf.println("ADD " + diff.b);
  if (diff cast JsonDiff.Different) // also covers DifferentType
    ret with buf.println(diff.a + " -> " + diff.b);
  if (diff cast Map<O>) {
    for (O key, O value : diff) {
      if (!value instanceof JsonDiff.Different
        || !key instanceof JsonDiff.GatheredKey)
        if (key cast JsonDiff.GatheredKey) {
          buf.println(key.key + "=" + key.value);
        } else
          buf.println(key);
      temp buf.indent();
      nicelyFormatJsonDiff(value, buf);
    }
    ret;
  }
  buf.println(sfu(diff)); // something unknown
}