% Slot 'age' has a 'when_accessed' facet.

person subclass of living_thing with
    age:
	[when_accessed:
		rule year1993
			if	the year_of_birth of Person is Year
			then	the age of Person is 1993 - Year];

tom instance of person with
    year_of_birth : 1972;

fred instance of person;

askable year_of_birth "when was this person born? ";