It really is! I wonder why they didn't specify the namespaces they were using instead of littering it with namespace and scope operators?

The intention is to avoid any kind of name clashes; I don't think this really inhibits readability.

Yep, it's not the most attractive parser ever. I chose to use Bison in order to save a lot of time/work and to take advantage of its GLR implementation; unfortunately its interaction with C++ isn't pleasant (due to the use of a union type for the symbol values).

That looks fairly clean for a Yacc parser. I do wish there was a more modern, friendlier parser-generator; yacc C++ tends to leak AST objects on error unless you use a pool allocator.