I must be doing something stupid. How can I process a sequence of sequences? I expected to be able to use nested invocations of SEQ_FOR_EACH, but I'm not getting the results I expect. Here's my code: #include <boost/preprocessor.hpp> #define INNER(R, DATA, ELEM)\ **ELEM**, #define OUTER(R, DATA, ELEM)\ << BOOST_PP_SEQ_FOR_EACH_R(R, INNER, ~, ELEM) >>, BOOST_PP_SEQ_FOR_EACH(OUTER, ~, ((1)(2)) ((3)(4)) ) I expected something like: << **1**, **2**, >>, << **3**, **4**, >>, But instead I'm getting: << BOOST_PP_SEQ_FOR_EACH_M(3, (INNER, ~, (1)(2) (nil))) BOOST_PP_SEQ_FOR_EACH_M(4, (INNER, ~, (2) (nil))) >>, << BOOST_PP_SEQ_FOR_EACH_M(4, (INNER, ~, (3)(4) (nil))) BOOST_PP_SEQ_FOR_EACH_M(5, (INNER, ~, (4) (nil))) >>, I've tried both msvc-8.0 and gcc-3.4. Can anybody see my mistake? -- Eric Niebler Boost Consulting www.boost-consulting.com