public async Task <LessonExtension> FetchLesson(Lesson lesson)
{
ILessonExtensionCrawler crawler;
if (lesson.Version)
{
crawler = m_New;
}
else
{
crawler = m_Old;
}
var ext = lesson.Extension();
var ann = crawler.GetAnnouncements(lesson).Then(ext.Merge);
var doc = crawler.GetDocuments(lesson).Then(ext.Merge);
var ass = crawler.GetAssignments(lesson).Then(ext.Merge);
await Task.WhenAll(ann, doc, ass);
return(ext);
}