using namely understandable placeholders

[DarkTrick@…]

Current situation: At the time, placeholders have rather generic names (_a,_1,_r1,...). That leads to code like this (from the error handling tutorial):

<< _4 what failed? << construct<std::string>(_3, _2) iterators to error-pos, end

This is hardly self-explanatory code.

Suggestion: change placeholder names to humanly understandable phrases, where possible.

[Nikita Kniazev <nok.raven@…>]

I do not think it is a good idea to spoil the qi namespace with different aliases, but you can do it yourself if you want to.

qi::_2_type const it_end_placeholder;
qi::_3_type const it_err_placeholder;
qi::_4_type const what_failed_placeholder;