namespace qi = boost::spirit::qi;
class my_grammar : public qi::grammar<string::const_iterator, string()>
{
public:
  my_grammar() : base_type(start) {
    using qi::_1;
    using qi::char_;
    start = *(char_ - qi::lit(','));
  }
  qi::rule<string::const_iterator, string()> start;
};