Minor "to do" items are identified in the source code with "TODO..." comments.

Major items to do:

* Use the new versioning infrastructure to identify all errors specific to
  - profile
  - extension
  - version
  - stage
  
* Add semantics for all new grammar (1.2 through 4.3)

* Replace the preprocessor with a better one.

* Verify/fix thread safety of the parse context.