class Book
{
    Library[] libraries;

    void RemoveFromLibrary(Library lib)
    {
        libraries.remove(lib);
        if (IsInLibrary(lib) && lib.HasBook(this)) // prevents call-loop
            lib.RemoveBook(this);
    }

    bool IsInLibrary(Library lib)
    {
        return libraries.Contain(lib);
    }
}


class Library
{
    Book[] books;

    void RemoveBook(Book bk)
    {
        books.remove(bk);
        if (HasBook(bk) && bk.IsInLibrary(this)) // prevents call-loop
            bk.RemoveLibrary(this);
    }

    bool HasBook(Book bk)
    {
        return books.Contain(bk);
    }
}