...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::external_transform — A placeholder for use as the second parameter for proto::when
to indicate that the rule's transform is specified externally.
// In header: <boost/proto/transform/when.hpp> struct external_transform { };