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

svoid nicelyFormatJsonDiff(O diff, StringBuilderWithIndent buf) {
  if (diff == null) ret "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.a);
  if (diff cast JsonDiff.Different) // also covers DifferentType
    ret with buf.println(diff.a + " -> " + diff.b);
  buf.println(sfu(diff)); // something unknown
}