=== Pairs ===
In Avatar Logic, data consists mainly of ordered pairs.

A pair can also be thought of as a directed edge between two nodes.

For example:

  (a, b)          pair with two concrete elements `a` and `b`
                  an edge from `a` to `b`
  (a, q'(b))      pair with a 1-avatar `q'(b)`
                  an edge from `a` to `q'(b)`
  (a, p(b))       pair with application `p(b)`
                  an edge from `a` to `p(b)`

This is different from normal logic, where predicates are used instead.

The benefit of using pairs is that can think about the underlying
data as a matrix with columns and rows for each term.
The data of pairs also can also be thought of as a graph,
with nodes and edges, where each edge is a pair.

The left argument `A` of the pair `(A, B)` can be thought of as
the input, while the right argument `B` can be thought of as the output.

What pairs mean depends on the interpretation of what you are modeling.