docs: protected org.mozilla.javascript.xml.XMLLib.Factory
          getE4xImplementationFactory()
      {
          // Must provide default implementation, rather than abstract method,
          // so that past implementors of ContextFactory do not fail at runtime
          // upon invocation of this method.
          // Note that the default implementation returns null if we
          // neither have XMLBeans nor a DOM3 implementation present.
  
          if (isDom3Present()) {
              return org.mozilla.javascript.xml.XMLLib.Factory.create(
                  "org.mozilla.javascript.xmlimpl.XMLLibImpl"
              );
          }
          return null;
      }