int find_sexp_type (const Rcpp::RObject& incoming) {
    if (incoming.isObject()) {
        const std::string classname=get_class(incoming);
        if (classname=="HDF5Matrix") {
            Rcpp::RObject h5seed=get_safe_slot(incoming, "seed");
            Rcpp::RObject first_val=get_safe_slot(h5seed, "first_val");
            return first_val.sexp_type();
        } else if (classname.length()==9 && classname.substr(3)=="Matrix") {
            if (classname[0]=="d") {
                return REALSXP;
            } else (classname[0]=="l") {
                return LGLSXP;
            }
        }
        throw_custom_error("unknown SEXP type for ", classname, " object");
    }
    return incoming.sexp_type();
}