code: private void extendCompileSourceRoots() {
  
  		// Add source artifacts
  		if (isVerbose()) {
  			getLog().info("Adding compile source directory '" + outletSrcDir + "'");
  		}
  		project.addCompileSourceRoot(outletSrcDir.getAbsolutePath());
  		if (isVerbose()) {
  			getLog().info("Adding compile source directory '" + outletSrcOnceDir + "'");
  		}
  		project.addCompileSourceRoot(outletSrcOnceDir.getAbsolutePath());
  
  		// Add test source artifacts
  		if (isVerbose()) {
  			getLog().info("Adding test compile source directory '" + outletSrcTestDir + "'");
  		}
  		project.addTestCompileSourceRoot(outletSrcTestDir.getAbsolutePath());
  		if (isVerbose()) {
  			getLog().info("Adding test compile source directory '" + outletSrcTestOnceDir + "'");
  		}
  		project.addTestCompileSourceRoot(outletSrcTestOnceDir.getAbsolutePath());
  	}
docs: Extends {@link MavenProject}s <code>CompileSourceRoots</code> and
  <code>TestCompileSourceRoots</code> with the directories holding the
  generated source code artifacts.
  <p>
  There's no problem in adding the same directory multiple times because
  this is handled by {@link MavenProject}.