code: protected Resource loadXtendFile(GeneratedMetamodel metamodel) {
  		if (xtendFile == null) {
  			final String extension = getExtensionName(metamodel);
  			try {
  				URI uri = getXtendFileLocation(metamodel);
  				if (uri != null) {
  					URIConverter uriConverter = metamodel.eResource().getResourceSet().getURIConverter();
  					if (uriConverter.exists(uri, null)) {
  						InputStream in = uriConverter.createInputStream(uri);
  						try {
  							XtendResourceParser parser = new XtendResourceParser();
  							xtendFile = parser.parse(new InputStreamReader(in), extension + '.'
  									+ XtendFile.FILE_EXTENSION);
  							fireXtendFileLoaded();
  						} finally {
  							if (in != null)
  								in.close();
  						}
  					}
  				}
  			} catch (ClasspathUriResolutionException ignored) {
  				// no xtend file found
  			} catch (Exception e) {
  				logger.error("Could not parse " + extension, e);
  			}
  		}
  		return xtendFile;
  	}
docs: Expects an Xtend file named <code>MyDsl</code>PostProcessor.ext with an extension with signature
  process(xtext::GeneratedMetamodel) in the same folder as the grammar file.
  
  @param metamodel
  the metamodel to augment
  @return the xtendFile to execute