This is an intermediate level between AST and CIVL-IR (model).

* tree representation of a program, like AST
* whole program represented by one tree (no notion of "translation unit", etc.)
* every variable is declared in one and only one place
* no prototypes --- you can call a function even before it is declared or defined
* no side-effects.  assignments are statements.   function calls are statements.
* some simplification of types?

This is a good level on which to perform transformations.