how to parse list of idents

Hello,

in symbol_def_parameter, i would like to parse:
"parameter identA" and/or
"parameter identA, identB" , ...

But the formular dont differ between space: ' ' and ','
So, Now my question is, how to declare symbol_def_parameter
in correct way?

----%<----

  symbol_def_parameter =
           (symbol_parameter >
           (symbol_alpha) |
          *(symbol_alpha) > qi::char_(',') > symbol_alpha)
          ;

  symbol_alpha =
       qi::char_("a-zA-Z_") >>
      *qi::char_("a-zA-Z0-9_")
      ;

   symbol_parameter = no_case["parameter"];


  BOOST_SPIRIT_DEBUG_NODE(symbol_parameter);
  BOOST_SPIRIT_DEBUG_NODE(symbol_def_parameter);

}

qi::rule<Iterator, std::string()> symbol_alpha;

qi::rule<Iterator, Skipper>
symbol_def_parameter;


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