If this hook predicate succeeds it must instantiate the Action
argument to the atom
fail to make the operation fail
retry to tell Prolog to retry the operation or
to make the system generate an exception. The action
only makes sense if this hook modified the environment such that the
operation can now succeed without error.
existence_errorexception. The hook is intended to implement alternatives to the built-in autoloader, such as autoloading code from a database. Do not use this hook to suppress existence errors on predicates. See also unknown and section 2.13.