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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
            proto::basic_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.