Error's in grammar structure

Hello,

I get this (g++ 5.4.0):

yymain.cc:213: required from 'client::dbase_grammar<Iterator, 
Skipper>::dbase_grammar() [with Iterator = 
__gnu_cxx::__normal_iterator<const char*, 
std::__cxx11::basic_string<char> >; Skipper = 
client::dbase_skipper<__gnu_cxx::__normal_iterator<const char*, 
std::__cxx11::basic_string<char> > >]'


source code:

             expression =
                     term.alias() [ e_ast = qi::_1 ] // <-- line 213
                     >> *(
                       ('+' >> term [ e_ast += qi::_1 ] )
                     | ('-' >> term [ e_ast -= qi::_1 ] ))
                     ;

what is wrong?

Jens

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most 
engaging tech sites, SlashDot.org! http://sdm.link/slashdot