=== Inequality ===
Avatar Logic supports inequality in rules.

For example:

  (a, foo)
  (X, Y) :- (Y, X), X != foo.

Since `foo != foo` is false, the pair `(foo, a)` will not be generated.

Variables can be used as patterns:

  (X, Y) :- (Y, X), X != F'(Z).

Inequality is often used to terminate rules,
that otherwise would expand infinitely.