private IEnumerable<IGrouping<int, IGrouping<BookTitle, Book>>> GroupBooks(BookCollection bookCollection)
{
return bookCollection
.GroupBy(x => x.Accept(y=>y))
.GroupBy(x => x.Count())
.OrderBy(x => x.Key);
}
public BookTitleCollection GetBooks(BookCollection bookCollection)
{
var titles = bookCollection
.GroupBy(x => x.Accept(y=>y))
.Select(x => x.Key);
return new BookTitleCollection(titles);
}
