[boost] Preprocessor question

Hi, Suppose I have the following macro: #define DO(x)\ BOOST_PP_CAT(STEP3, BOOST_PP_CAT(STEP2, BOOST_PP_EXPAND(STEP1 x))) #define STEP2STEP1 Now, DO(5) should IMO expand into STEP35. For some reason, VC71 expands it into STEP3 5. Where does the space come from? Can one CAT two tokens, and get a space between them? Is it a bug in VC71 or is it supposed to be like this? If I use BOOST_PP_SEQ_CAT outside, it does what I expect. Can anybody explain why? (the above exersize may not make sence out of context. I don't think the context is relevant, although I can provide it if necessary) Thanks in advance. Regards, Arkadiy