Dynamic loop counter on x3

Hello,

I'm using spirit x3. I'd like to use `repeat(N)[a]` and want to
determin N dynamically. I found the document for qi. I tried to do the
same thing the last part of
http://www.boost.org/libs/spirit/doc/html/spirit/qi/reference/directive/repeat.html
on x3.

Here is the code I wrote:

http://melpon.org/wandbox/permlink/JfQQpSb2cS7an8jP

#include <iostream>
#include <boost/spirit/home/x3.hpp>

namespace x3 = boost::spirit::x3;

int main() {
    std::string str;
    int n = 5;
    std::string source = "\x0bHello World";
    x3::parse(
        source.begin(), source.end(),
        x3::char_[([&n](auto& ctx){ n = _attr(ctx); })] >>
        x3::repeat(n)[x3::char_][([&str](auto& ctx){ str = _attr(ctx);})]);
    std::cout << n << ',' << str << std::endl;  // will print "11,Hello World"
}

I got the following output:
11,Hello

It seems that the value 'n' is not updated.

Am I missing something or x3 doesn't support dynamic repeat count yet?

Thanks,
Takatoshi Kondo

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