// In header: <boost/proto/expr.hpp>
template<typename Tag, typename Args, long Arity = Args::arity> 
struct expr {
  // types
  typedef Tag                                   proto_tag;         
  typedef Args                                  proto_args;        
  typedef mpl::long_< Arity >                   proto_arity;       
  typedef proto::default_domain                 proto_domain;      
  typedef proto::basic_expr< Tag, Args, Arity > proto_grammar;     
  typedef expr                                  proto_base_expr;   
  typedef expr                                  proto_derived_expr;
  typedef typename Args::childN                 proto_childN;        // For each N in [0,max(Arity,1)).
  // member classes/structs/unions
  template<typename Signature> 
  struct result {
    // types
    typedef unspecified type;
  };
  // public static functions
  template<typename... A> static expr const make(A const &...);
  // public member functions
  expr & proto_base();
  expr const & proto_base() const;
  template<typename A> unspecified operator=(A &);
  template<typename A> unspecified operator=(A const &);
  template<typename A> unspecified operator=(A &) const;
  template<typename A> unspecified operator=(A const &) const;
  template<typename A> unspecified operator[](A &);
  template<typename A> unspecified operator[](A const &);
  template<typename A> unspecified operator[](A &) const;
  template<typename A> unspecified operator[](A const &) const;
  template<typename... A> unspecified operator()(A const &...);
  template<typename... A> unspecified operator()(A const &...) const;
  // public data members
  proto_childN childN;  // For each N in [0,max(Arity,1)).
  static const long proto_arity_c;  // = Arity;
};