It might be desirable to organize the knowledge in the Algernon KB in a more modular way.
We will exploit the backlink from the set Diseases to the individual known diseases. The :taxonomy form asserts explicit links from sets to the specified elements, Flu and Plague. However, in case we learn about more diseases, we provide a rule to assert the backlink from an isa relation.
(tell '((:rules Diseases
((isa ?d Diseases) -> (member Diseases ?d)))))
In general, the ``upward'' isa relation, pointing from an element to a set it belongs to, is not backlinked to the ``downward'' member relation. Some sets (e.g. Things or People) could have large numbers of elements. The access-limited philosophy of Algernon discourages inferences that scan over large numbers of elements.
We can then formulate a single general-purpose rule that says that a person has a disease if he or she has every symptom of the disease. (Obviously, neither of these approaches is very realistic.)
(tell '((:rules People
((has-disease ?x ?d)
<-
(member Diseases ?d)
(:all-paths ((symptom ?d ?s)) ((has-symptom ?x ?s)))))))
Within this approach, we can specify the symptoms caused by a disease, not in a rule associated with the set People, but as part of the declarative description of the disease itself.
(tell '((symptom Flu Fever) (symptom Plague High-Fever)
(symptom Flu Nausea) (symptom Plague Nodules)))