=== Equality ===
An equality is an expression of the following form:

  p(a) = b

Equalities are used to substitute sub-expressions in application.

For example, `p(a)` is substituted with `b` when `p(a) = b`.

In Avatar Logic, equalities are inferred from,
but stored as separated facts, from pairs, e.g. `(a, b)` where `b : p`.

This allows one to control substitution behavior.

For example, a 1-avatar `territory` might be used to describe
countries that share the same capital. When asking
what the capital is of the country, it is convenient to lift the equality:

  capital(X) = Y :- capital(territory'(X)) = Y.

When a "has" relation `p(a) => q'(b)` where `uniq q`,
it is lifted to `p(a) = q'(b)` automatically.

Equalities are lifted into "has" relations automatically.