#include <boost/fusion/container/map.hpp>
#include <boost/fusion/sequence/intrinsic/at_key.hpp>
#include <boost/fusion/sequence/intrinsic/value_at_key.hpp>

struct some_tag {};

typedef
	boost::fusion::map
	<
		boost::fusion::pair< some_tag, int >
	>
	my_map
	;

typedef
	boost::fusion::result_of::value_at_key
	<
		  my_map
		, some_tag
	>::type
	return_type
	;

return_type foo( my_map & m )
{
	return boost::fusion::at_key< some_tag >( m ) = 5;
}

int main()
{
	my_map m;

}