You are trainee at a very formal library in London. Every book must be indexed in one of two giant catalogs. Catalog A contains the citation of every book that makes no reference to itself. Catalog B contains the citation of every book that does contain a reference to itself.
The elderly librarian, who is assigned to your group of trainees, now asks the group: "In which volume would you index catalog B?" A perceptive student suggests placing a reference for 'Catalog B' in volume A, since B does not make any reference to itself. The librarian asks again: "Then where would you index Catalog A?"