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

http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/users_g... 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