[boost] boost::proto overloads of operator()

Hi, I'm trying to get boost::proto to turn udt(v1,v2) into a parse tree. Up to now I haven't got past the compiler asking for an udt::operator()(int,int). I can probably define such an operator for my UDT and return a proto::expr tree, but I was hoping in proto providing overloads so that I don't have to do it for all my types. I've looked into expr.hpp and there seem to be something related to this, but I don't seem to be able to make use of it. What is the best way to achieve what I'm trying? Thanks a lot, Maurizio