=== 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)