Superclass of exceptions which indicate that an unknown kind of
entity was encountered. This situation can occur if the language
evolves and new kinds of constructs are introduced. Subclasses of
this exception may be thrown by visitors to indicate that the
visitor was created for a prior version of the language.
API Note:
A common superclass for exceptions specific to different
kinds of unknown entities allows a single catch block to easily
provide uniform handling of those related conditions.