docs: public static XMLStreamException missingRequired(final XMLStreamReader reader, final Set<?> required) {
          final StringBuilder b = new StringBuilder();
          Iterator<?> iterator = required.iterator();
          while (iterator.hasNext()) {
              final Object o = iterator.next();
              b.append(o.toString());
              if (iterator.hasNext()) {
                  b.append(", ");
              }
          }
          return new XMLStreamException("Missing required attribute(s): " + b, reader.getLocation());
      }