The rules of the librarians of time and space:
2. Books must be returned no later than the date shown.
3. Do not meddle with the nature of causality.
from the Discworld books by Terry Pratchett
"You see, I don't believe that libraries
should be drab places where people
sit in silence, and that's been the main reason
for our policy of employing wild animals as librarians. ......."
from the Monty Python skit Gorilla Librarian