Hi Eric,
Proto's users guide has been behind the times for a while. No longer.
More recent and powerful features are now documented. Feedback welcome.
Sub-domains:
============
http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/users_guide/front_end/customizing_expressions_in_your_domain/subdomains.html
proto::default_domain
as a sub-domain of
every other domain."proto::default_domain
as a super-domain of
every other domain."