18.1.2.4 Visible Modifications by Language Extensions

Implementations that extend the language by providing additional mutator functions (or additional behavior for existing mutator functions) must document how the use of these extensions interacts with equivalence tests and hash table searches.

Implementations that extend the language by defining additional acceptable equivalence tests for hash tables (allowing additional values for the :test argument to make-hash-table) must document the visible components of these tests.