[boost] [proto]are there 2 ways to define generators?

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-05-31 10:14:37

In:

   html/boost_proto/user_s_guide/extending_proto.html

there's:
<--------- cut here -----------
  namespace boost { namespace proto
{
     // A specialization of generate<> that causes expression in the
     // calculator_domain to be wrapped in the calculator<> wrapper.
     template< typename Expr >
     struct generate< calculator_domain, Expr >
     {
         typedef calculator< Expr > type;

         static type make( Expr const & expr )
         {
             return type( expr );
         }
     };
}}

This instructs Proto to wrap all expressions in the calculator_domain in 
the calculator<> wrapper.
 >--------- cut here -----------
OTOH, in:

   libs/xpressive/proto/example/calc3.cpp

there's:
<--------- cut here -----------
// Tell proto how to generate expressions in the calculator_domain
struct calculator_domain
   : proto::domain<proto::generator<calculator_expression> >
{};
 >--------- cut here -----------

which suggests to me that there's 2 different ways to wrap expressions
in a particular domain.  Is that right? If so, what are the advantages
of one over the other method?