This code is confusing to me. Why are the pdf_textbooks and the html_textbooks set to the same list? Is it that these are just ids, and it is okay for an HTML textbook to have the same ids as PDF textbooks? In any other language I'd want to have two different lists even if the contents were the same. Is this safe in Python? If I remove one of the PDF textbooks, will it remove the corresponding HTML textbook too?