copy constructor of symbols does not do a deep copy?

If I run code like this

    boost::spirit::qi::symbols<char, double> syms1;
    boost::spirit::qi::symbols<char, double> syms2( syms1 );
    syms1.at("x") = 0.0;
    syms2.at("x") = 99.5;
    std::cout << "In syms1, x=" << syms1.at("x") <<
        " while in syms2, x=" << syms2.at("x") << std::endl;

I get the output "In syms1, x=99.5 while in syms2, x=99.5".  Why didn't x stay at 0.0 in syms1?