cannot write std::map via ostream_iterator?

Can you tell me why the program below does not compile? GCC says:

$ make mapout.o
c++ -O2 -c mapout.cpp
/usr/include/g++/bits/stream_iterator.h: In member function `
   std::ostream_iterator<_Tp, _CharT, _Traits>& std::ostream_iterator<_Tp,

   _CharT, _Traits>::operator=(const _Tp&) [with _Tp = std::pair<const
   std::string, std::string>, _CharT = char, _Traits =
std::char_traits<char>]
   ':
[...]
mapout.cpp:21: instantiated from here
/usr/include/g++/bits/stream_iterator.h:141: error: no match for
'operator<<'
   in '*this->std::ostream_iterator<std::pair<const std::string,
std::string>,
   char, std::char_traits<char> >::_M_stream << __value'

I understand that ostream_iterator::operator=(rhs) calls
operator<<(ostream&, rhs) and as far as I'm concerned that operator is
declared, right after the typedef. But when two compilers disagree with
me, I usually like to admit I'm wrong.

It smells like a const problem, but it must be possible to write
std::pair<const string, string> to an ostream via an iterator. (Or
perhaps it has to do with the context of the template instantiation?)

Am I expected to derive my own iterator? If so, why?

The workaround of course is std::transform with a function to convert the
pair to a string, but that's not efficient in terms of human or machine
cycles.

Many thanks,

--jkl

[snip]
#include <iostream>
#include <fstream>

#include <iterator>
#include <algorithm>
#include <map>
#include <string>

using namespace std;

typedef map<string,string> map_ss;

ostream&
operator<<( ostream& os, const map_ss::value_type& elem );

int
main( int argc, char *argv[] )
{
    map_ss ml;

    copy( ml.begin(), ml.end(), ostream_iterator<map_ss::value_type>(cout)
);

    return 0;
}
[pins]

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