[boost] [proto]howto generate extended tree top down instead of bottom up?

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-07-07 10:50:52

http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/users_guide/expression_extension/expression_generators.html

contains:

   we want all expressions that involve the calculator placeholders to be
   calculators

which means, IIUC, that calulator expressions are built bottom up.
IOW, to build a calculator expressions, you must start with a
proto terminal calculator expression.  However, I would like
to start from a non-terminal expression (e.g. any binary
or unary expression)  and convert all the nodes to calculator 
expressions.  What's the simplest way to do this?  I've tried
understanding trans::construct and other trans templates; however,
I can't figure it out so far :(

TIA for any clues.

-regards,
Larry