Boost
  C++ Libraries
  
  ...one of the most highly
  regarded and expertly designed C++ library projects in the
  world.
 — Herb Sutter and Andrei
  Alexandrescu, C++
  Coding Standards
boost::proto::logical_not::impl
// In header: <boost/proto/traits.hpp> template<typename Expr, typename State, typename Data> struct impl : proto::pass_through<logical_not>::template impl<Expr, State, Data> { };