Unsupported:
- internal dtds
- external entities
- ANY tags

ToDo:
- rest of the SAX callbacks
  ignorable whitespace
  processing instruction
- ordering constraints
- code-size reduction

Done:
- PCDATA <![CDATA[ ]]>
- parsing of tags and attributes
- validation of tags, attributes, and enumerations
- defaults/fixed
- Validation of input
- unique IDs
- PCDATA
- &lt; etc...
- comments
- command line support for packages, specifying base element, etc.
