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## Struct template nary_expr

boost::proto::nary_expr — A metafunction for generating n-ary expression types with a specified tag type, a grammar element for matching n-ary expressions, and a PrimitiveTransform that dispatches to the `proto::pass_through<>` transform.

## Synopsis

    // In header: <boost/proto/traits.hpp>

    template<typename Tag, typename... A>
    struct nary_expr :  proto::transform< nary_expr<Tag, A...> > {
      // types
      typedef proto::expr< Tag, proto::listN< A... > >       type;
      typedef proto::basic_expr< Tag, proto::listN< A... > > proto_grammar;

      // member classes/structs/unions
      template<typename Expr, typename State, typename Data>
      struct impl :
         proto::pass_through<nary_expr>::template impl<Expr, State, Data>
      {
      };
    };

## Description

Use `proto::nary_expr<proto::_, proto::vararg<proto::_> >` as a grammar element to match any n-ary expression; that is, any non-terminal.