Hi, this post originates from thread "concatenation of power of negative number" in comp.std.c++ (http://tinyurl.com/asn95). Consider this file: ---- #include <boost/preprocessor/cat.hpp> BOOST_PP_CAT(1e, -1) ---- According to Paul Mensonides' reply in the thread, it should be eventually interpreted as <1e> ## <-> <1> thus the pp-number "1e" is concatenated with "-" obtaining the pp-number "1e-" and then "1" is appended as a separate token. The final result would thus be "1e- 1" (notice the blank between "-" and "1"). However, if run into Wave, the program above produces the error message: error: pasting the following two tokens does not give a valid preprocessing token: "e" and "-" Who's right? I'd agree with Paul Mensonides, because "1e" is a valid pp token which is to be formed before applying the ## operator. But my understanding is quite limited in this area so I commit myself to the Gurus. Ganesh