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_MAX_ARITY — Controls the maximum number of child nodes an expression may have.
// In header: <boost/proto/proto_fwd.hpp>
BOOST_PROTO_MAX_ARITY
        BOOST_PROTO_MAX_ARITY defaults to 10. It may be set higher or lower, but not
        lower than 3. Setting it higher will have a negative effect on compile times. 
      
        See also BOOST_PROTO_MAX_FUNCTION_CALL_ARITY.