Package the code. Move parser macros to their own file so that they

Package the code.  Move parser macros to their own file so that they
can be loaded during compilation.

Restructure the top-level parser to allow passing the DTD down to the
element parser for entity expansion.

Implement attribute-value normalization.

Add restrictions required for rejection of non-wf documents: no "--"
in comments; no duplicate attribute names.

Change names of DTD structures to reflect their keywords.

Fix various minor bugs, discovered either by the test suite or by the
compiler.