[lmi] callback.hpp vs boost::function

[Vadim Zeitlin]

 Hello,

 while fixing this file to compile with g++ 4.0 (for the curious, it was
just a missing typename keyword: compiler can't know that the expression
boost::xxx<T>::type is a type for any template parameter T, so you have to
tell it that it is), I couldn't avoid asking myself why does this callback
class exist at all and why is not boost::function used instead? It looks it
does exactly the same thing but boost::function is, of course, much more
general (even if it's not needed now it surely doesn't harm) and, another
big advantage, it's alreay written, tested and debugged, so why reinvent it?

 Please let me know if I'm missing something,
VZ