docs: protected URL transformURL(URL link, ReferenceContext references) {
  		if (URISchemeType.FILE.isURL(link)) {
  			File filename = FileSystem.convertURLToFile(link);
  			if (Strings.isEmpty(filename.getName())) {
  				// This is a link to the local document.
  				final String anchor = transformURLAnchor(filename, link.getRef(), references);
  				final URL url = FileSystemAddons.convertFileToURL(filename, true);
  				if (!Strings.isEmpty(anchor)) {
  					try {
  						return new URL(url.toExternalForm() + "#" + anchor); //$NON-NLS-1$
  					} catch (MalformedURLException e) {
  						//
  					}
  				}
  				return url;
  			}
  			// This is a link to another document.
  			final String extension = FileSystem.extension(filename);
  			if (isMarkdownFileExtension(extension)) {
  				filename = FileSystem.replaceExtension(filename, ".html"); //$NON-NLS-1$
  				final String anchor = transformURLAnchor(filename, link.getRef(), null);
  				final URL url = FileSystemAddons.convertFileToURL(filename, true);
  				if (!Strings.isEmpty(anchor)) {
  					try {
  						return new URL(url.toExternalForm() + "#" + anchor); //$NON-NLS-1$
  					} catch (MalformedURLException e) {
  						//
  					}
  				}
  				return url;
  			}
  		}
  		return null;
  	}