newbie Question about the macro BOOST_JOIN

Hi,all, In file suffix.hpp there are lines: #define BOOST_JOIN( X, Y ) BOOST_DO_JOIN( X, Y ) #define BOOST_DO_JOIN( X, Y ) BOOST_DO_JOIN2(X,Y) #define BOOST_DO_JOIN2( X, Y ) X##Y why donot we define such macro instead: #define BOOST_JOIN( X, Y ) X##Y thanks in advance.