docs: public static String requireSingleAttribute(final XMLStreamReader reader, final String attributeName)
              throws XMLStreamException {
          final int count = reader.getAttributeCount();
          if (count == 0) {
              throw missingRequired(reader, Collections.singleton(attributeName));
          }
          requireNoNamespaceAttribute(reader, 0);
          if (!attributeName.equals(reader.getAttributeLocalName(0))) {
              throw unexpectedAttribute(reader, 0);
          }
          if (count > 1) {
              throw unexpectedAttribute(reader, 1);
          }
          return reader.getAttributeValue(0);
      }