=== Roles ===
In Avatar Logic, a role is kind of like a type,
but is also kind of like a function, or sometimes like a set.

For example:

  london : city       the city London

When you have a pair:

  (a, london)

Avalog adds the following expressions:

  city(a) = london      the city of `a` "is" London
  city(a) => london     the city of `a` "has" a London

These expressions are used for further inference.
The expresssion `city(a) = london` means that when you write
`city(a)` anywhere, it can be substituted with `london`.