 language
that is used to talk about the world (such as KIF, CGs, SQL,
Express, Epistle, or even English).

The concept of Variable is used to describe certain languages,
but it is not necessary in all kinds.  The more primitive notion
is _coreference_, i.e. expressing the fact that two different
signs (words, symbols, images, etc.) refer to the same entity.
There are many different ways of showing coreference:

 1. Pronouns and other indexicals in English and other natural
    languages.

 2. Connecting li