19    static std::vector<std::string> allowed_log_levels{
"none", 
"error", 
"warn",
 
   20                                                       "info", 
"debug", 
"all"};
 
   21    static auto* allowed_log_levels_vals =
 
   22        new TCLAP::ValuesConstraint<std::string>(allowed_log_levels);
 
   24    return TCLAP::ValueArg<std::string>(
 
   25        "l", 
"log-level", 
"the verbosity of logging messages", 
false,
 
   31        allowed_log_levels_vals);