Contains definition of proto::generator<> class template and friends that end users can use to generate domain-specific expression wrappers.

namespace boost {
  namespace proto {
    struct default_generator;
    template<template< typename > class Extends> struct generator;
    template<template< typename > class Extends> struct pod_generator;
    struct by_value_generator;
    template<typename First, typename Second> struct compose_generators;
  }
}