docs: public void validateDuplicateDefaultErrorPageConfiguration(String newLocation, ConfigItem<String> priorLocationItem) {
         
         String priorLocation = priorLocationItem.getValue();
  
         if (priorLocation == null) {
             if (newLocation == null) {
                 return; // Same null Location; ignore
             } else {
                 // Different Locations: null vs non-null; check the source
             }
         } else if (newLocation == null) {
             // Different Locations: non-null vs null; check the source
         } else if (priorLocation.equals(newLocation)) {
             return; // Same non-null Location; ignore
         }
  
         ConfigSource priorSource = priorLocationItem.getSource();
         ConfigSource newSource = getConfigSource();
         
         if (priorSource == ConfigSource.WEB_XML) {
             if (newSource == ConfigSource.WEB_FRAGMENT) {
                 if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled()) {
                     Tr.debug(tc, "Default error page with location {0} in web.xml overrides default error page with location {1} in web-fragment.xml in {2}.",
                              priorLocation, newLocation, getLibraryURI());                   
                 }                
             } else {
                 // web.xml vs web.xml: Should not be possible; ignore.
             }
             
         } else if (priorSource == ConfigSource.WEB_FRAGMENT) {
             // If the prior source is a web-fragment.xml, we have started
             // processing fragments, and any subsequent source must also
             // be a fragment.
  
             String priorLibraryURI = priorLocationItem.getLibraryURI();
             String newLibraryURI = getLibraryURI();
             
             if (!priorLibraryURI.equals(newLibraryURI)) {
                 errorMessages.add(nls.getFormattedMessage("CONFLICT_DEFAULT_ERROR_PAGE_WEB_FRAGMENT_XML",
                                                           new Object[] { priorLocation, priorLibraryURI, newLocation, newLibraryURI },
                                 "Multiple default error pages with different locations in one or more web-fragment.xml."));               
             } else {
                 // Same library; should not be possible; ignore
             }
         }
      }