=== Rules ===
A rule in Avatar Logic is similar to rules in Prolog.
For example:
(X, Y) :- (Y, X), X : capital, Y : country.
The left side `(X, Y)` is the Head and the right side is the Body of rule:
Head :- Body.
The Head is true if the Body is true.
When adding facts, the rules can infer new facts:
england : country
london : capital
(england, london)
-----------------
(london, england)