Struct template is

Struct template is_expr

boost::proto::is_expr — A Boolean metafunction that indicates whether a given type T is a Proto expression type.

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

template<typename T> 
struct is_expr :  mpl::bool_<true-or-false> {
};

If T is an instantiation of proto::expr<> or is an extension (via proto::extends<> or BOOST_PROTO_EXTENDS()) of such an instantiation, proto::is_expr<T>::value is true. Otherwise, proto::is_expr<T>::value is false.