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::term — A type sequence, for use as the 2nd parameter to the 
          proto::expr<> class template.
template<typename Arg0> struct term { // types typedef Arg0 child0; static const long arity; };
            A type sequence, for use as the 2nd parameter to the 
            proto::expr<> class template. The
            types in the sequence correspond to the children of a node in an expression tree.