Spirit.Qi specific Phoenix placeholders

_1, _2... , _N

Nth attribute of p

_val

The enclosing rule's synthesized attribute.

_r1, _r2... , _rN

The enclosing rule's Nth inherited attribute.

_a, _b... , _j

The enclosing rule's local variables (_a refers to the first).

_pass

Assign false to _pass to force a parser failure.

template <typename Iterator>
bool parse_numbers(Iterator first, Iterator last, std::vector<double>& v)
{
    using qi::double_;
    using qi::phrase_parse;
    using qi::_1;
    using ascii::space;
    using phoenix::push_back;
    using phoenix::ref;

    bool r = phrase_parse(first, last,

        //  Begin grammar
        (
            double_[push_back(ref(v), _1)]
                >> *(',' >> double_[push_back(ref(v), _1)])
        )
        ,
        //  End grammar

        space);

    if (first != last) // fail if we did not get a full match
        return false;
    return r;
}

why using _1, (using qi::_1) instead of _2 (using qi::_2).

Which parts of boost should I read before reading spirit.
Any suggestion is welcome. Thank you.