const ex arg_pt = arg.subs(rel, subs_options::no_pattern);
if (!arg_pt.info(info_flags::integer) || arg_pt.info(info_flags::positive))
throw do_taylor(); // caught by function::series()
const ex arg_pt = arg.subs(rel, subs_options::no_pattern);
if (!arg_pt.info(info_flags::integer) || arg_pt.info(info_flags::positive))
throw do_taylor(); // caught by function::series()