conditional config file loading using boost::program_options

jinserk (1)
for example, I have a config file like this:
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num = 2

[a0]
x = 0
y = 1
z = 2

[a1]
x = 0
y = 1
z = 2


if num = 3, another group a3 will be appended after the config file.

How can I read this file using boost::program_options?

I've tried this code using exception,

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fstream file("config.txt", fstream::in);

boost::program_options::options_description desc;
boost::program_options::variables_map vm;

try {
    desc.add_options()
        ("num", boost::program_options::value<int>(&num), "number")
        ;

    boost::program_options::store(boost::program_options::parse_config_file(file, desc), vm):
}
catch (exception& e) {
    int num;
    if (vm.count("num")) {
        num = vm["num"].as<int>();

        for (int i = 0; i < num; i++)

         // desc options a0, a1, a2, ... adding

    store again 
    .......
}


however, store() didn't store the loaded parameters in vm when unknown options are found in the file.

Please help!
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